The structure of a plant leaf. Features of leaf structure. The structure of a plant leaf, types of arrangement of leaf plates, photosynthesis and transpiration. Leaf is a lateral organ of a shoot.

SHEET
a plant organ initially specialized for photosynthesis, i.e. nutrition of the body, but in the course of evolution sometimes loses this function or acquires additional functions. Of all the creations of nature, green, i.e. containing chlorophyll, the leaf is the most important structure for life on Earth. Without it, humans and other organisms could not exist. The atmospheric supply of oxygen is replenished by the continuous release of this gas from the leaves of green plants. Leaves absorb up to 400 billion tons of carbon dioxide per year, while binding 100 billion tons of carbon in organic compounds. Since carbon is the main component of all living organisms, leaves serve as the primary source of food and vital vitamins for humans, as well as for all wild and domestic animals, without which humans could not exist. See also CARBON CYCLE. Leaves provide people with more than just oxygen and food. In the tropics, for example, people still live in huts covered with palm leaves. All over the world, wood remains one of the most important building materials, which could not have been formed if there were no leaves on the trees. If we ignore purely utilitarian needs, we should remember that leaves make our lives more pleasant and comfortable. Delicious and tonic drinks are prepared from them, for example, regular tea from the leaves of the tea bush or “mate” from the leaves of the Paraguayan holly, a shrub that grows along the banks of rivers in Argentina, Paraguay and southern Brazil. Smoking tobacco leaves (Nicotiana tabacum) helps many people relax. Potent medicines are obtained from the leaves of various plants, such as coca, foxglove, and belladonna. Aloe vera leaves contain substances that cure some dermatitis, relieve pain from radiation and sunburn and soften the skin. Some leaves, which have a pleasant aroma, are used directly as seasonings or serve as raw materials for the production of fragrant extracts. This is exactly what is used for, for example, the leaves of basil, bay leaves, marjoram, thyme, lavender and peppermint.

Fiber for making ropes is obtained from the leaves of Sansevieria cylindrica and Agave sisalana; mats, bedspreads and hats are woven from the leaves of some other species.

A typical leaf consists of three parts - the blade, the petiole, and the stipules - small leaf-like structures at the base of the petiole. The main part is a plate, usually thin, flat and green. However, in some plants its color is different, for example, dark red in Iresine herbstii, variegated in coleus and croton, or silver in Santolina chamaecyparissus. Sometimes the surface of the leaf is pubescent, i.e. covered with hairs - outgrowths of outer cells. The petioles of some leaves, such as celery and rhubarb, are very large and are eaten. Sometimes there are no petioles at all, and the leaf blade is attached directly to the stem. Such leaves are called sessile. They can be found, for example, in Diervilla sessilifolia. The stipules of most plants are small, but sometimes they are quite comparable in size to the leaf blade, as in garden peas or Japanese chaenomeles. In some cases, such as in black locust (Robinia pseudoacacia), the stipules are transformed into thorns.
Leaf shape- one of the distinctive features of the plant species. The sheet can be simple or complex, i.e. consisting of several leaves, depending on whether it has one plate or several. Thus, birches, beeches, elms, oaks and plane trees have simple leaves, while horse chestnuts, white acacia, rose hips, ailanthus and walnuts have complex leaves. Compound leaves are pinnately and palmately compound. In the first case, the leaves are located in two opposite rows along a common axis, as, for example, in white acacia and walnut, and in the second, they move away from one point, as, say, in horse chestnut or clover.
Leaf sizes vary widely depending on the taxon and even within the same plant species. They can reach a length of 20 m, for example in the Raphia ruffia palm tree, which grows in tropical Africa and Madagascar. Vegetable asparagus (Asparagus officinalis var. altilis), casuarina equisetifolia and French comb (Tamarix gallica) have very small leaves. In most cases, the leaves are wide and flat, but sometimes they are cylindrical, like onions, needle-shaped, like pine trees, or scale-like, like cypress trees. There are leaves linear (in cereals), rounded (in nasturtium), ovoid (in the frame), heart-shaped (in linden), lanceolate (in willow), etc. Sometimes the so-called heterophyly (“multiple leaves”) - leaves of different shapes are formed on the same plant; for example, sassafras has five variants. Leaves with smooth edges are called entire leaves. Among trees, such leaves can be seen, for example, in dogwood, lilac, rhododendron, eucalyptus, imbricated oak, loosestrife and virginiana. In many cases, the edges of the leaf blade are lobed, dissected, serrated, and notched. For example, red oak leaves are pinnately lobed with spiny protrusions of veins at the tops of the lobes, while white oak leaves are pinnately lobed or smoothly notched without sharp corners. In most plants, the leaf arrangement is alternate, or spiral: the leaves, like the buds with lateral shoots, extend one at a time from each node, either on one or the other side of the stem. An example is all birches, elms, hazel trees, oaks and walnuts. In some species, in particular maples, viburnum and dogwood, leaves, buds and side shoots are located oppositely - on opposite sides of each node. When three or more leaves extend from a node, the leaf arrangement is called whorled. In any case, the leaves move away from the stem so as to minimally shade each other. They form a kind of “leaf mosaic” in space, designed to capture as much sunlight falling on the plant as possible.

SIMPLE AND COMPLEX LEAVES. A leaf is called simple or complex depending on whether it has one plate or several. In the second case, the leaf can be pinnately compound, if its constituent leaflets are arranged in two rows on a common axis, or palmately compound, when they emerge from one point - the tip of the petiole.

TYPES OF LEAF ARRANGEMENT. There are three main types of leaf arrangement: opposite, alternate (spiral) and whorled. In the first case, a leaf emerges from each node on two opposite sides of the stem. In the second case, the leaves move away from the nodes one at a time - first on one side, then on the other side of the stem. If three or more leaves extend from a node, their arrangement is called whorled.

Leaf blade. A typical leaf blade consists of a thin layer of surface cells - the epidermis and an underlying multilayered internal tissue - mesophyll. The mesophyll is penetrated by a system of veins. A thin section of a leaf under a microscope shows that the outside of the epidermis is covered with a cuticle - a film consisting of a waxy cutin. This film is interrupted in places by inclusions of pectin-like substances. Through such areas, the leaf can absorb substances containing nitrogen, phosphorus, potassium and other elements necessary for the nutrition and normal functioning of the plant from solutions falling on its surface. The cuticle and epidermis protect the inner cells from rapid drying out, and the thickness of these outer layers often indicates the species' adaptation to its environment. Thus, in pines and other narrow-leaved evergreens, the powerful cuticle very effectively slows down evaporation, especially in winter, when the frozen soil contains little water available for the roots. The cuticle and epidermis are permeated with tiny holes - stomata, the number of which is not equal on both sides of the leaf. Each stomata is a gap between two bean-shaped guard cells, which, slightly changing their shape, open or close it. This regulates the rate of transpiration, i.e. loss of water by the plant. When the stomata are open, water vapor evaporates through them into the atmosphere and this ensures the upward movement of new portions of water with salts dissolved in it from the roots to the leaves and other parts of the shoots. Gas exchange between the plant and the environment also occurs through stomata. Guard cells react sensitively to the light level: when it increases, the stomata open wider; when it gets dark, the stomatal fissure becomes narrower. Thus, stomatal gas exchange and transpiration are much more intense during the day than at night. The epidermis of the leaf also contains specialized stomata - hydathodes, which secrete water in the form of droplets. This process is called guttation. Its intensity is maximum when a lot of water is absorbed and evaporation is slow. Contrary to popular belief, dew drops observed on the grass on a summer morning are the result of guttation, and not the condensation of atmospheric moisture. The main part of the leaf is mesophyll. Directly under the upper (sometimes also under the lower) epidermis there are one or several cylindrical layers perpendicular to the surface of the so-called leaf. palisade cells - palisade parenchyma. Each of these cells contains numerous miniature bodies - chloroplasts, containing the green pigment chlorophyll, which captures solar energy and converts it into chemical energy. This process, called photosynthesis, produces sugars from carbon dioxide in the atmosphere and water from the soil. Under the palisade parenchyma there are large cells that make up the spongy parenchyma. Free spaces between them (intercellular spaces) facilitate the diffusion of gases inside the leaf. Spongy parenchyma contains fewer chloroplasts, and photosynthesis is not as intense here as in palisade parenchyma. Veins penetrating the leaf, i.e. vascular-fibrous bundles that conduct water and nutrients are surrounded by a bundle sheath, or lining, of thin-walled, compactly located cells. The upper part of the vein consists of xylem, formed by vessels and tracheids, and the lower part of phloem, represented mainly by sieve tubes. Through the xylem, water with dissolved mineral salts moves from the roots to the leaf blades, and through the phloem, photosynthetic products - organic substances - are sent from the leaf to all organs of the plant. There are two main types of leaf venation - reticulate, when the veins branch and connect to each other, and parallel, when they run parallel to each other. The first type is typical for dicotyledonous flowering plants - geranium, tomato, maple, oak, etc.; the second - for monocots, i.e. iris, lilies, cereals (for example, corn, bamboo, wheat), etc. There are various deviations from this pattern and transitional types of venation.

LEAF STRUCTURE
Photosynthesis. The main function of the leaf is photosynthesis, during which sugars are formed from water and carbon dioxide due to solar energy. From these sugars, substances specific to them are formed in various plant organs, which are necessary, for example, for growth, lignification of cells, ripening of fruits and seeds, etc. Sugars are stored in reserve so that they can be used if necessary. Thus, the green leaf is an organ on which the provision of plants with organic substances entirely depends. To grow, plants need the same organic substances as animals (proteins, fats, carbohydrates, etc.), but only photosynthesis allows them to be obtained from inorganic compounds. All living beings that are not capable of photosynthesis depend directly or indirectly on green plants for their nutrition. The process of photosynthesis is very complex, and here we will consider it only in the most general terms. Typically, carbon dioxide enters the leaf from the atmosphere through the stomata, spreads through the intercellular space, passes through the cell wall and is absorbed by the fluid filling the cells. The carbon dioxide that gets inside the chloroplasts and the water that is always present here enter into a series of reactions that produce various intermediate products, ultimately sugars, in particular the water-soluble sugar glucose and the product of its polymerization, starch. Further, proteins are formed from sugars through certain reactions with nitrogen and sulfur compounds (coming mainly from the soil). All other compounds necessary for the body, such as cellulose, lignin, fats, oils, etc., are ultimately built from sugars. See also PHOTOSYNTHESIS.
Development and fall of leaves. Leaves develop from areas of rapidly growing stem tissue - the meristem, located in the buds at the apex of the stem and in the shoot nodes. Before the buds open, not yet dissected leaf primordia are formed from the meristem in the form of a tubercle or roller on the growth cone - the so-called. leaf primordia. As the bud opens, their cells begin to rapidly divide, grow, and specialize until the leaf is fully formed. As the leaf develops in its axil, i.e. At the top of the angle between the leaf and the section of the stem going up from it, a new bud is almost always formed. New shoots may arise from such axillary buds next year. Unlike roots and stems, a leaf is a temporary organ. Having reached full development, after some time it dies and falls off. In deciduous species in the temperate climate zone, this occurs every autumn. Before this, the plant hormone abscisin II stimulates the formation at the base of the leaf petiole (or its blade, if the leaf is sessile) of a special layer of specialized tissue, the so-called. separating layer. It consists mainly of spongy parenchyma, i.e. thin-walled cells loosely connected to each other, therefore, under the influence of its own weight, as well as external influences, such a leaf relatively easily breaks off from the stem. In evergreen species, the foliage is also renewed, but each leaf lives for several years, and the leaves do not fall all at once, but one by one, so that outwardly these changes are invisible. This phenomenon is widespread in tropical plants, on which at any time of the year you can see leaves in different stages of development: some are ready to fall, others are just unfolding, and others are passing the peak of maturity and metabolic activity.
Autumn leaf color. The leaves become especially brightly colored in the fall in some geographic regions, for example, the northeastern and northwestern United States, southeastern continental Asia, and southwestern Europe. In Northern Europe, where winters are mild and rainy, the leaves turn mostly dirty yellow and brownish before falling. Fall leaf color depends largely on the type of plant, but is also affected by weather conditions and soil type. Before the leaves fall, nutrients are transferred from them to the stems and roots. The formation of chlorophyll stops, and its remains are quickly destroyed by sunlight. As a result, yellow pigments, mainly xanthophylls and carotenes, become visible. They are present in the leaves throughout the growing season, but are masked by green chlorophyll in spring and summer. The orange, red and purple tones of autumn foliage are caused by other pigments - anthocyanins, which, unlike yellow pigments, appear only in the fall, and their amount depends on the weather. If the air temperature drops sharply to a level of 0-7 ° C, more sugars and tannins remain in the leaf, and as a result, the synthesis of anthocyanins is activated. Thus, if autumn is sunny, dry and cool, the leaves of many trees delight the eye with bright red, yellow, orange and crimson tones. If the autumn is cloudy, and the nights are warm and less sugar is synthesized in the leaves, and a significant proportion of it passes from them to the stem, then the formation of anthocyanins is weak and the color of the leaves becomes predominantly dull yellow. One of the most beautiful species in autumn is the sugar maple (Acer saccharum), whose leaves turn deep yellow, golden orange and bright red. In red maple (A. rubrum) they turn red, and in Norway maple (A. platanoides) and silver maple (A. saccharinum) they become golden-yellow in color. The autumn crown of Liquidambar styraciflua cannot but cause admiration: in the same tree it can shimmer in different shades of purple, scarlet, yellow and green. Other tree species that turn red in autumn include Nyssa sylvatica, Oxydendrum arboreum, American scarlet oak (Quercus coccinea) and swamp oak (Q. palustris). The leaves of Florida dogwoods (Cornus florida) and pink dogwoods (C. florida rubra) turn bright crimson earlier than most other trees. Among the shrubs, winged euonymus (Euonymus alatus), various types of barberry (Berberis spp.) are famous for their bright autumn leaves. ) and American mackerel (Cotinus americanus).
Specialized leaves. Leaves can specialize in various ways, losing their typical appearance, structure and even functions. Examples of such leaves are the tendrils of many legumes, which allow plants to cling to supports, the spines of cacti, in which the processes of photosynthesis have moved to green fleshy stems, the protective bud scales of trees, as well as bracts - scale-like covering leaves on the pedicels of many species. Sometimes the leaves surrounding flowers and entire inflorescences are bright and conspicuous, such as the white or red spathes of arum (calla, anthurium) or the red, white and pink apical leaves of poinsettia (Euphorbia pulcherrima). They can easily be mistaken for petals, while the real flowers of these species can be relatively small and inconspicuous. The American agave (Agave americana) leaves are very thick and fleshy - they store water and nutrients. Other plants with a pronounced storage function of leaves include various purslanes (genus Portulaca) and sedums (genus Sedum). Their leaves contain mucilaginous colloidal substances that effectively bind water and slow down its evaporation in the arid habitat typical of these “leaf succulents.” At the so-called Insectivorous plant leaves are turned into traps for small arthropods. Thus, in the Venus flytrap (Dionaea muscipula), the halves of the leaf, covered at the edges with spines protruding upward, can rotate relative to the midrib. When an insect lands on a leaf blade, these halves slam shut like a book, and the victim finds itself in a trap. Its body decomposes under the action of enzymes secreted by leaf glands, and the decomposition products are absorbed by the plant. The pitcher plant (Nepenthes) has modified leaves in the shape of a pitcher. An insect that crawls in cannot get out; it drowns and is digested in the liquid secreted by the glands at the bottom of the jug. The leaves of many plants are dangerous for warm-blooded animals. Thus, rooting sumac (Toxicodendron radicans) contains an oily substance, which, when it gets on the skin, causes severe inflammation (dermatitis). The leaves of some species of astragalus (genus Astragalus) accumulate selenium, which is poisonous to animals. Cattle that have eaten a large amount of these leaves become ill with selenosis, from which they sometimes die. Leaves are poisonous, for example, in plants such as Dieffenbachia picta, lily of the valley (Convallaria majalis), azaleas and rhododendrons (genus Rhododendron), and Kalmia latifolia. see also POISONOUS PLANTS.

Collier's Encyclopedia. - Open Society. 2000 .

Synonyms:

A leaf is a vegetative organ of plants and is part of a shoot. The functions of the leaf are photosynthesis, water evaporation (transpiration) and gas exchange. In addition to these basic functions, as a result of idioadaptations to various living conditions, leaves, changing, can serve the following purposes.

Accumulation of nutrients (onions, cabbage), water (aloe);
protection from being eaten by animals (cactus and barberry spines);
vegetative propagation (begonia, violet);
catching and digesting insects (sundew, Venus flytrap);
movement and strengthening of weak stems (pea tendrils, vetch);
removal of metabolic products during leaf fall (in trees and shrubs).

General characteristics of the plant leaf

The leaves of most plants are green, most often flat, usually bilaterally symmetrical. Sizes range from a few millimeters (duckweed) to 10-15 m (palm trees).

The leaf is formed from the cells of the educational tissue of the growth cone of the stem. The leaf primordium is differentiated into:

Leaf blade;
the petiole by which the leaf is attached to the stem;
stipules.

Some plants do not have petioles; such leaves, unlike petiolate ones, are called sedentary. Not all plants have stipules either. They are paired appendages of various sizes at the base of the leaf petiole. Their shape is varied (films, scales, small leaves, spines), and their function is protective.

Simple and compound leaves distinguished by the number of leaf blades. A simple leaf has one blade and falls off entirely. The complex one has several plates on its petiole. They are attached to the main petiole with their small petioles and are called leaflets. When a compound leaf dies, first the leaflets fall off, and then the main petiole.

The leaf blades are varied in shape: linear (cereals), oval (acacia), lanceolate (willow), ovate (pear), arrow-shaped (arrowhead), etc.

The leaf blades are pierced in different directions by veins, which are vascular-fibrous bundles and give the leaf strength. The leaves of dicotyledonous plants most often have reticulate or pinnate venation, while the leaves of monocotyledonous plants have parallel or arcuate venation.

The edges of the leaf blade can be solid; such a leaf is called whole-edged (lilac) or with notches. Depending on the shape of the notch, along the edge of the leaf blade, leaves are distinguished as serrated, serrated, crenate, etc. In serrated leaves, the teeth have more or less equal sides (beech, hazel), in serrated leaves, one side of the tooth is longer than the other (pear), crenate - have sharp notches and blunt protuberances (sage, budra). All these leaves are called whole, since their grooves are shallow and do not reach the width of the blade.

In the presence of deeper grooves, the leaves are lobed when the depth of the groove is equal to half the width of the blade (oak), separate - more than half (poppy). In dissected leaves, the notches reach the midrib or the base of the leaf (burdock).

Under optimal growth conditions, the lower and upper leaves of the shoots are not the same. There are lower, middle and upper leaves. This differentiation is determined in the kidney.

The lower, or first, leaves of the shoot are the bud scales, the outer dry scales of the bulbs, and the cotyledon leaves. The lower leaves usually fall off as the shoot develops. The leaves of basal rosettes also belong to the grass roots. Median, or stem, leaves are typical of plants of all species. The upper leaves usually have smaller sizes, are located near flowers or inflorescences, are painted in various colors, or are colorless (covering leaves of flowers, inflorescences, bracts).

Types of sheet arrangement

There are three main types of leaf arrangement:

Regular or spiral;
opposite;
whorled.

In the next arrangement, single leaves are attached to the stem nodes in a spiral (apple tree, ficus). In opposite case, two leaves in a node are located one opposite the other (lilac, maple). Whorled leaf arrangement - three or more leaves at a node envelop the stem in a ring (elodea, oleander).

Any leaf arrangement allows plants to capture the maximum amount of light, since the leaves form a leaf mosaic and do not shade each other.

Cellular structure of the leaf

The leaf, like all other plant organs, has a cellular structure. The upper and lower surfaces of the leaf blade are covered with skin. Living colorless skin cells contain cytoplasm and a nucleus and are located in one continuous layer. Their outer shells are thickened.

Stomata are the plant's respiratory organs

The skin contains stomata - slits formed by two guard, or stomatal, cells. Guard cells are crescent-shaped and contain cytoplasm, nucleus, chloroplasts and a central vacuole. The membranes of these cells are thickened unevenly: the inner one, facing the gap, is thicker than the opposite one.

A change in the turgor of guard cells changes their shape, due to which the stomatal fissure is open, narrowed or completely closed, depending on environmental conditions. So, during the day the stomata are open, but at night and in hot, dry weather they are closed. The role of stomata is to regulate the evaporation of water by the plant and gas exchange with the environment.

Stomata are usually located on the lower surface of the leaf, but they can also be on the upper surface; sometimes they are distributed more or less evenly on both sides (corn); In aquatic floating plants, stomata are located only on the upper side of the leaf. The number of stomata per unit leaf area depends on the plant type and growth conditions. On average there are 100-300 of them per 1 mm2 surface, but there can be much more.

Leaf pulp (mesophile)

Between the upper and lower skins of the leaf blade is the leaf pulp (mesophil). Beneath the top layer there are one or more layers of large rectangular cells that have numerous chloroplasts. This is a columnar, or palisade, parenchyma - the main assimilation tissue in which photosynthesis processes take place.

Under the palisade parenchyma there are several layers of irregularly shaped cells with large intercellular spaces. These layers of cells form spongy, or loose, parenchyma. Spongy parenchyma cells contain fewer chloroplasts. They perform the functions of transpiration, gas exchange and nutrient storage.

The pulp of the leaf is penetrated by a dense network of veins, vascular-fibrous bundles, which supply the leaf with water and substances dissolved in it, as well as remove assimilants from the leaf. In addition, the veins perform a mechanical role. As the veins move away from the base of the leaf and approach the top, they become thinner due to branching and the gradual loss of mechanical elements, then sieve tubes, and finally tracheids. The smallest branches at the very edge of the leaf usually consist only of tracheids.

Diagram of the structure of a plant leaf

The microscopic structure of the leaf blade varies significantly even within the same systematic group of plants, depending on different growing conditions, primarily on lighting and water supply conditions. Plants in shaded areas often lack palisade parenchyma. The cells of the assimilative tissue have larger palisades; the concentration of chlorophyll in them is higher than in light-loving plants.

Photosynthesis

In the chloroplasts of pulp cells (especially columnar parenchyma), the process of photosynthesis occurs in the light. Its essence lies in the fact that green plants absorb solar energy and create complex organic substances from carbon dioxide and water. This releases free oxygen into the atmosphere.

Organic substances created by green plants are food not only for the plants themselves, but also for animals and humans. Thus, life on earth depends on green plants.

All oxygen contained in the atmosphere is of photosynthetic origin; it accumulates due to the vital activity of green plants and its quantitative content is maintained constant due to photosynthesis (about 21%).

By using carbon dioxide from the atmosphere for the process of photosynthesis, green plants thereby purify the air.

Evaporation of water by leaves (transpiration)

In addition to photosynthesis and gas exchange, the process of transpiration occurs in the leaves - the evaporation of water by the leaves. The main role in evaporation is played by the stomata; the entire surface of the leaf partly takes part in this process. In this regard, a distinction is made between stomatal transpiration and cuticular transpiration - through the surface of the cuticle covering the epidermis of the leaf. Cuticular transpiration is significantly less than stomatal transpiration: in old leaves it is 5-10% of total transpiration, but in young leaves with a thin cuticle it can reach 40-70%.

Since transpiration occurs mainly through stomata, where carbon dioxide also penetrates for the process of photosynthesis, there is a relationship between the evaporation of water and the accumulation of dry matter in the plant. The amount of water that is evaporated by a plant to build 1 g of dry matter is called transpiration coefficient. Its value ranges from 30 to 1000 and depends on growth conditions, type and variety of plants.

To build its body, the plant uses an average of 0.2% of the water passed through, the rest is spent on thermoregulation and transport of minerals.

Transpiration creates a suction force in the leaf and root cells, thereby maintaining the constant movement of water throughout the plant. In this regard, the leaves are called the upper water pump, in contrast to the root system - the lower water pump, which pumps water into the plant.

Evaporation protects the leaves from overheating, which is of great importance for all plant life processes, especially photosynthesis.

Plants in dry areas and in dry weather evaporate more water than in humid conditions. In addition to the stomata, the evaporation of water is regulated by protective formations on the leaf skin. These formations are: cuticle, waxy coating, pubescence from various hairs, etc. In succulent plants, the leaf turns into spines (cacti), and its functions are performed by the stem. Plants in humid habitats have large leaf blades and no protective formations on the skin.

Transpiration is the mechanism by which water evaporates from plant leaves.

When evaporation is difficult in plants, guttation- release of water through stomata in a drop-liquid state. This phenomenon occurs in nature usually in the morning, when the air is approaching saturation with water vapor, or before rain. In laboratory conditions, guttation can be observed by covering young wheat seedlings with glass covers. After a short period of time, droplets of liquid appear at the tips of their leaves.

Excretion system - leaf fall (leaf fall)

A biological adaptation of plants to protect themselves from evaporation is leaf fall - the massive fall of leaves during the cold or hot season. In temperate zones, trees shed their leaves during the winter, when the roots cannot draw water from the frozen soil and frost dries out the plant. In the tropics, leaf fall occurs during the dry season.

Preparation for shedding leaves begins when the intensity of life processes weakens in late summer - early autumn. First of all, chlorophyll is destroyed; other pigments (carotene and xanthophyll) last longer and give the leaves an autumn color. Then, at the base of the leaf petiole, parenchyma cells begin to divide and form a separating layer. After this, the leaf is torn off, and a mark remains on the stem - a leaf scar. By the time the leaves fall, the leaves become old, unnecessary metabolic products accumulate in them, which are removed from the plant along with the fallen leaves.

All plants (usually trees and shrubs, less often herbs) are divided into deciduous and evergreen. In deciduous plants, leaves develop during one growing season. Every year, with the onset of unfavorable conditions, they fall off. The leaves of evergreen plants live from 1 to 15 years. The dying off of some old leaves and the appearance of new leaves occurs constantly, the tree appears to be evergreen (conifers, citrus fruits).

Shoot growth occurs exogenously leaf primordium. It is located below the tip of the shoot and has the appearance of an oval tubercle. The cells of the leaf primordium divide in all directions, thus the leaf grows both in thickness and in height. As soon as growth in thickness stops, the leaf takes on a flat appearance.

There are two parts of the leaf primordium: apical(top) and basal(lower). The apical growth of the leaf primordium is limited and does not last long. When the top of the leaf stops growing, the base continues to grow. In other words, acropetal growth ends and basipetal growth begins. Thus, the apical meristem completes its growth function, and the intercalary meristems begin their development.

The leaf blade and petiole directly develop from the upper part of the leaf primordium, and the base of the leaf and stipules develop from the lower part. Sometimes a set of leaf parts is already formed in the bud, and when it enters from the bud, the already formed parts grow and their anatomical structures are differentiated. The petiole is one of the last to grow.

Note 1

It is worth noting that not all leaves have a petiole.

The leaf blade increases in size quite uniformly. The leaf is monosymmetrical in most plants. The leaf has two surfaces - dorsal (dorsal) and ventral (ventral). The dorsal surface in the bud is located inside, thus adjacent to the stem, and in the developed leaf it is at the top. The abdominal one, on the contrary, is located outside in the bud, and below in the developed leaf.

Modification of leaves (Metamorphoses)

Leaves are modified depending on the growing conditions of the plant, and in connection with the adaptation of plants to certain functions. Spines, scales, tendrils, phyllodes, growth of hairs on the leaf are all modifications of the leaves.

The spines of plants perform two functions: less evaporation of water (cactus in the desert) and protection from animals. The spines have different locations on the stem. For example, in barberry the thorn is located under the leaf, in hawthorn it is in the axil of the leaf. The leaf blade of a cactus has turned into a thorn. In astragalus, the rachis of a compound leaf has changed into a spine; in acacia, the stipule has changed.

The shoots of vines have adapted for support in order to occupy a certain position in space. A similar function is performed by the modified leaves into tendrils of the pea, they help the plant move due to their tenacity.

The scales of bulbous plants play a special role; they accumulate nutrients. Also, the covering scales of buds, bulbs, and rhizomes perform a protective function.

Trapping devices as modifications of leaves are characteristic of insectivorous plants. The leaves are modified and resemble water lilies, urns, slamming sticky plates. The sticky hairs of the sundew feed the plants, the insect lands on the sticky surface, the leaf closes and the animal begins to decompose under the action of enzymes. This modification occurred due to the fact that the plant grew on soil deficient in minerals.

Bag-shaped leaf modifications are found in an epiphytic plant of a tropical rainforest. Water and humus accumulate in such formations. As a result, adventitious roots are formed in the leaves, supplying the plants with moisture.

Phyllodes cover the shoots of the club moss, i.e. are outgrowths on the stem. They come in green, etc. They can photosynthesize, or they can carry sporangia in the form of sacs in which spores are formed. Acacia also has phyllodes. The petiole of the acacia tree is transformed into a flat leaf-like formation.

The hair and waxy coating on the leaves is adapted to retain moisture and delay the evaporation process. The shiny surface of the ficus reflects light rays, which contributes to less evaporation of water by the plant.

The denticles along the edges of plants are adapted to express the processes of photosynthesis and transpiration. Thus, condensation occurs, which leads to the formation of dew.

Pheromones, poisons, aromatic oils, crystallization minerals produced by leaves can repel pests. The petals pollinate insects.

Note 2

Thus, modification of leaves is capable of adapting plants to the environment and being resistant to unfavorable conditions.

Leaf arrangement of leaves

Leaf arrangement, or phyllotaxis- This is the order in which the leaves are placed on the stem, thus reflecting the symmetry in the structure of the shoot. The position of the leaf depends on the order of the laid primordia on the growth cone. Most plants have leaves located directly on the stems and branches, so that general rules for their arrangement can be established. At first glance, the leaves appear to be arranged randomly. But if you look closely at the leaves, you will see that the leaves sit in pairs, one against the other, this arrangement is called opposite. On some plants, leaf pairs alternate so that they cross each other, this is called decussate. If there are three leaves on one node, which alternate with each other, and maybe even $4-10$ or more leaves, then the arrangement is called ringed. If on stems with ring-shaped leaves, the leaves sit on top of each other, then you get several verticals and lines parallel to each other, which are called orthostic. If you draw a line from the very first bottom to the nearest sheet, then from the second to the nearest, etc. until the end, a spiral line is formed, then this leaf arrangement is called spiral.

In each polynomial leaf arrangement, in addition to the main spiral, secondary steeper spirals are observed. They are called parastiches.

Picture 1.

If three or more leaves extend from a node, the leaf arrangement is called whorled. With a rosette leaf arrangement, the leaves are in a rosette, i.e. a bunch of leaves are arranged in a circle from one common center.

Sheet mosaic

Definition 1

Sheet mosaic- this is a kind of arrangement of the leaves of a plant in one plane, in such a way as to ensure the least shading of each other’s leaves. The leaves are directed perpendicular to the direction of the light rays. All this is the result of the uneven growth of petioles and leaf blades, which reach towards the light and fill every illuminated gap. Leaf mosaic is formed in absolutely any leaf arrangement - opposite, whorled, rosette, alternate or crosswise opposite.

People write poems and songs about them, admire them in spring, summer and autumn, and look forward to their appearance in winter. They are a symbol of life and the rebirth of nature, a delicate robe that pleases the eye and gives pure oxygen to all living things on earth. These are leaves - what we see every day and what not a single plant, or even our entire planet, can live without.

- Yellow leaves are circling over the city, falling under our feet with a quiet rustle...

- Maple leaf, maple leaf, I dreamed about you in the middle of winter...

- Green leaves ringing to all those who were in love...

What are leaves, why are they needed, why do they turn yellow in the fall and grow again in the winter, what colors and shapes they come in - you will learn all this and much more from this publication.

Functions of leaves, their role in plant life

Speaking in dry scientific language, a leaf is one of the most important organs of a plant, the main function of which is to participate in the process of photosynthesis.

[!] Photosynthesis is the conversion of solar energy into organic compounds inside a plant. Simply put, through photosynthesis, plants obtain food from the sun's rays.

In addition, with the help of leaves, the plant breathes and evaporates moisture (releases dew).

As you can see, without green covers, plant life would be impossible, but not only plants depend on leaves. With the help of these peculiar lungs, the plant neutralizes carbon dioxide and releases oxygen, which is necessary for people, animals, and insects, that is, for all living things on the planet.

In general, the sheet consists of several parts:

The base is the place of attachment to the stem;
Stipule - leaf-like elements at the base, in some cases falling off after the leaf has fully opened;
Petiole - a continuation of the main vein of the leaf blade, connecting the leaf and stem;
The leaf blade is the wide part of the leaf that performs its main functions.

Since each plant is individual, and the leaves are very different, some parts may not be there. For example, stipules are often absent, and sometimes there is no petiole (in this case the leaves are called sessile or pierced). In addition, all parts can be of very different shapes, lengths and structures.

Classification and separation of the main parts helps botanists correctly identify a plant and determine which family, genus and order it belongs to.

Structure, types and shapes of the leaf plate

The leaf blade consists of an upper epidermis covered with a cuticle, a palisade layer, a spongy layer and a lower epidermis also covered with a cuticle. Each layer performs a specific function:

The cuticle and epidermis protect the plate from external influences and prevent excessive evaporation of water.

[!] Stomata are responsible for the process of retaining the necessary moisture inside the leaf - paired cells that can close and prevent moisture from evaporating. Stomata begin their work during drought, saving the plant from dehydration.

The palisade layer, also called the columnar tissue, is responsible for the process of photosynthesis. Chloroplasts, cells that color the surface of the leaf green, are also collected here.
Spongy tissue is the basis of the leaf plate. Its functions are gas exchange, absorption of carbon dioxide and release of oxygen, and photosynthesis.

The entire plate is permeated with conductive bundles, called veins, through which organic substances are delivered from the root to the leaf (water and minerals) and vice versa (sugar solution). In addition, the veins form a hard skeleton that protects the soft tissue from tearing.

Plate shapes

In general, all forms of leaves are divided into simple and complex, and complex ones into palmate, pinnate, bipinnate, trifoliate, pinnately cut, which, in turn, are divided into several more types. In total, botany has at least thirty-five varieties of forms.

Simple leaves consist of one leaf blade, and it can be of very different shapes: round, oval, diamond-shaped, elongated, and so on. The outline of the tip of the plate and the place of attachment of the petiole also differ.

Complex leaves are those that consist of several parts, both articulated on a common petiole (lobed, dissected, separate) and having their own separate petiole (palmated, pinnate, trifoliate).

[!] One of the signs of complex leaves is that they fall off at different times.

In addition to the general configuration of the leaf, its base (round, heart-shaped, trill-shaped, unequal, etc.) and apex (pointed, notched, tendril-shaped, blunt, etc.) are distinguished.

Edge shapes

The edge of a leaf, as well as its general shape, tells botanists whether a plant belongs to one species or another. Depending on the depth of the cut, the edges are divided into fingered or jagged (shallow notches), lobed, dissected and separated (deep notches). Smooth edges are called whole edges.

Types of venation

The venation pattern of the leaf blade can be very diverse and depends on the type of plant. In general, all types of venation are divided into two parts:

Several parallel veins pass through the leaf blade, but there is no central vein (parallel venation),
there is a main (central) vein, from which lateral veins branch (reticulate venation),
several curved veins diverging in the middle of the leaf and converging towards the edge (arcuate venation).

In turn, reticulate venation is divided into several subspecies.

Types of stipules and petioles

The stipule usually looks like a small, underdeveloped leaf located at the base of the leaf. They may fall off after the leaf has fully expanded or remain on the plant. Depending on the method of attachment to the petiole, stipules can be free, fused with the petiole, interpetiolar, trumpet-shaped, or encircling the base of the petiole.

Petioles can vary in cut shape: cylinder, half-cylinder, with a notch, and others. In addition, as mentioned above, there may be no petiole at all, in which case the leaf is attached directly to the stem.

As you can see, the plant world displays an amazing variety of forms, and there are millions of their combinations.

So, the scientific and botanical part is over, it's time to move on to the amazing facts about leaves.

How plants adapt to climate and other living conditions using leaves

Each plant is forced to acclimatize to weather conditions and also protect itself from external influences. All parts of the plant: roots, shoots, flowers and, of course, leaves, have adapted to various climatic phenomena: high or low temperature, drought or excessive humidity, lack or excess of sunlight. In addition, plants are threatened by people and animals, so many of them, in the process of evolution, have learned to repel attacks.

Let's consider how, with the help of its green cover, a plant resists an unfavorable environment.

Dry or humid climate:

The small size of the leaves and, accordingly, the small area of the leaf plate prevents excessive evaporation of water;
The leaves are usually thick and juicy - thus they accumulate the necessary moisture;
The leaf blades of many plants are covered with hairs, which also prevents evaporation;
A smooth waxy coating on the surface serves the same purpose.
Large leaves are a sign of plants in a tropical climate; due to the large size of the plate, the evaporation process occurs much more intensely.

Crassula, Saintpaulia, Philodendron

Windy areas:

The dissected, jagged shape of the edge allows air flows to pass freely, thanks to which gusts of wind do not injure the sheet.

Silver birch "Dalecarlian", monstera, palmate maple

Places with excess or insufficient sunlight:

If there is not enough sunlight, many plants can unfold their leaves so that as much sunlight as possible hits their surface;
Leaf mosaic is a phenomenon in which smaller leaves are located between larger ones. In this case, each leaf catches the sun's rays and participates in the process of photosynthesis;
Some plants that do not need a lot of sun filter light through special translucent windows located on the leaves.

Dandelion, ivy, fenestraria

aquatic plants– these representatives of the flora stand apart, because in order to survive they had to adapt not even to the climate, but to a completely different element – water:

The leaves of hydatophytes (plants completely submerged in water) are highly dissected. Thus, by increasing the surface area, the plant receives the necessary amount of oxygen;
Leaves floating on the surface of a reservoir do not have stomata on the back side of the leaf blade;
The large surface area of floating leaves prevents them from sinking by distributing the load.
Special microscopic protrusions and a waxy layer prevent water from penetrating into the leaf, preventing the plant from becoming infected with microorganisms and protozoan algae. Water is not absorbed into the surface, but flows down the sheet in drops, at the same time cleaning it from dust and dirt. This phenomenon is called the “lotus effect.”

Hornwort, Victoria amazonica, lotus

Protection from animals and people. Some plants, in the course of evolution, have learned to defend themselves from attacks:

The leaves produce strong-smelling pheromones and oils that repel animals;
The leaf blade can be covered with soft hairs or even hard spines that sting the aggressor.

Geranium, nettle, woolly chickweed

Unusual leaves

Nature has endowed certain types of plants with such an extravagant appearance that sometimes determining where the leaves are in front of us seems to be a difficult task.

Cactiformes settled in areas with arid climates, where the loss of every drop of water is tantamount to death. Evolutionary selection did its job - specimens with a minimum area of evaporation survived. Wide leaves are an unaffordable luxury for such living conditions. The entire external decoration of cacti, inhabitants of waterless wastelands, consists of compact protective leaves-spines.

Opuntia, Trichocerius, Schlumbergera

Other plants in arid regions, in order not to evaporate precious moisture, decided to abandon their leaves altogether. Or rather, they still have leaves, but only in the form of small, undeveloped scales. At the same time, shoots called cladodes or phyllocadia acquired the leaf shape and function of photosynthesis. Phyllocadia have adapted to their new role so much that they practically do not differ in appearance from an ordinary leaf, but in fact they are not such.

There is also the opposite option - what seems to be shoots are actually leaves. One example is the tendrils of creeping plants. In this case, the tendrils are the upper parts of the leaves, which have adapted to cling to support.

Ruscus, asparagus, pickled peas

Some of the most unusual leaves belong to tropical exotics. The hot, humid climate, the abundance of insects and animals forced plants to adapt to difficult living conditions and even become predators. Using sticky secretions or special bubbles on the leaves, predatory plants catch unwary insects and then suck the life juices out of them.

Another adaptation of tropical plants is a bag formed by fused planes of a leaf plate. This trap collects rainwater, the supply of which is, of necessity, used during periods of drought.

Sundew, pemphigus, Raffles dischidia

Leaves of different colors

What color are the leaves? At first glance, the answer to this question is very simple - green in summer, yellow and red in autumn. In fact, they can come in a variety of colors not only in autumn, but also at other times of the year. You can find green, yellow, red, silvery burgundy and even purple shades of color in the natural decoration of completely healthy plants. In addition to unusual pigmentation, the leaves of some, especially southern, plants have beautiful patterns and ornaments.

Zebrina, fittonia, caladium

Leaves are not only pleasing to the eye and necessary for the life of the planet, some of the leaves are also edible and, moreover, form a significant part of the human diet. In cooking, they are used as a vegetable component: spinach, chard, Chinese cabbage, Chinese cabbage, and as salad ingredients: arugula, sorrel, lettuce, and, of course, as seasonings: dill, parsley, basil, mint and so on.

Chinese cabbage, lettuce, basil

Answers on questions

At the end of the article there are answers to the most popular questions about leaves.

Why is the leaf flat?

This shape increases the area of the leaf blade, and, in turn, the larger surface area increases the number of cells involved in the process of photosynthesis.

What determines the sheet size?

The size and, accordingly, the surface area of the leaf depends on the habitat of the plant. The leaves of plants from dry areas are usually small, while those from humid areas are large. The fact is that the larger the leaf area, the more stomata on its surface and the more intense the evaporation of water occurs. Where there is often drought, in order to survive, plants try not to evaporate a lot of moisture, but in a tropical climate, the evaporation process, on the contrary, should be as intense as possible.

Why are the leaves green?

Chlorophyll, which is involved in the conversion of carbon dioxide into nutrients, is responsible for the green color of the leaf. The high chlorophyll content in the leaf blade gives the plants a fresh green hue.

[!] The chlorophyll of some plants is colored in other colors - red, brown, purple, so the leaves of such plants have corresponding shades.

Why do the leaves turn yellow?

In autumn, the chlorophyll in the leaves is destroyed and there is less of it. Due to the decrease in chlorophyll, the intensity of the green spectrum gradually decreases. The yellow and red pigments (xanthophyll, carotene, anthocyanin) contained in the leaf cells come to the fore.

[!] The leaves of some plants do not change color and fall green.

Why do leaves fall in autumn?

Seasonal changes in daylight hours and average daily temperatures forced plants to adapt to changing living conditions. By the onset of winter cold, most of the flora sheds its summer decorations and enters a state of suspended animation, commonly called hibernation. Metabolic processes in plant life systems practically stop. Leaves, so necessary in the summer for evaporating excess moisture and collecting life-giving sunlight, simply become unnecessary and fall off.

During spring and summer, leaves extract and process nutrients necessary for plant life. In the process of such processing, the green lungs of nature produce and accumulate metabolites - excess mineral salts, thus acting as a kind of filter. Over time, the deposits become more and more and in the fall the plant gets rid of the leaf, which ceases to be beneficial.

This is how nature works; nothing is wasted. Fallen leaves cover the ground from frost, protecting the soil. In the warm season, the carpet covering the soil gradually decomposes and overheats. Insects, bacteria and microorganisms process the resulting humus into nutritious soil for living plants, closing the cycle in nature.

a plant organ initially specialized for photosynthesis, i.e. nutrition of the body, but in the course of evolution sometimes loses this function or acquires additional functions. Of all the creations of nature, green, i.e. containing chlorophyll, the leaf is the most important structure for life on Earth. Without it, humans and other organisms could not exist. The atmospheric supply of oxygen is replenished by the continuous release of this gas from the leaves of green plants. Leaves absorb up to 400 billion tons of carbon dioxide per year, while binding 100 billion tons of carbon in organic compounds. It is these organic compounds formed in the leaves that serve as the primary source of food and vital vitamins for humans and all wild and domestic animals.

Leaves provide people with more than just oxygen and food. In the tropics, for example, people still live in huts covered with palm leaves. All over the world, wood remains one of the most important building materials, which could not have been formed if there were no leaves on the trees. If we ignore purely utilitarian needs, we should remember that leaves make our lives more pleasant and comfortable. They are used to prepare delicious and tonic drinks, for example, regular tea from the leaves of the tea bush or “mate” from the leaves of the Paraguayan holly, a shrub that grows along the banks of rivers in Argentina, Paraguay and southern Brazil. Smoking tobacco leaves ( Nicotiana tabacum) helps many people relax. Potent medicines are obtained from the leaves of various plants, such as coca, foxglove, and belladonna. Aloe vera leaves ( Aloe vera) contain substances that cure some dermatitis, relieve pain from radiation and sunburn and soften the skin. Some leaves, which have a pleasant aroma, are used directly as seasonings or serve as raw materials for the production of fragrant extracts. This is exactly what is used for, for example, the leaves of basil, bay leaves, marjoram, thyme, lavender and peppermint. From the leaves of Sansevieria cylindrical ( Sansevieria cylindrica) and Agave sisal ( Agave sisalana) fiber is obtained for making ropes; mats, bedspreads and hats are woven from the leaves of some other species.

Main parts and general characteristics.

A typical leaf has three parts: the blade, the petiole, and the stipules, small leaf-like structures at the base of the petiole. The main part is a plate, usually thin, flat and green. However, in some plants its color is different, for example, dark red in Herbst’s irezina, popular among gardeners ( Iresine herbstii), variegated in coleus (nettle), or silvery in santolina cypress ( Santolina chamaecyparissus), also known as cypress grass. Sometimes the surface of the leaf is pubescent, i.e. covered with hairs - outgrowths of outer cells.

The petioles of some leaves, such as celery and rhubarb, are very large and are eaten. Sometimes there are no petioles at all, and the leaf blade is attached directly to the stem. Such leaves are called sessile. They are characteristic, in particular, of Dierville sessifolia ( Diervilla sessilifolia), belonging to the honeysuckle family. The stipules of most plants are small, but sometimes they are quite comparable in size to the leaf blade, as in garden peas or Japanese chaenomeles. In some cases, for example in white acacia ( Robinia pseudoacacia), stipules transformed into spines.

Leaf shape is one of the distinguishing characteristics of a plant species. The sheet can be simple or complex, i.e. consisting of several leaves, depending on whether it has one plate or several. Thus, birches, beeches, elms, oaks and plane trees have simple leaves, while horse chestnuts, white acacia, rose hips, ailanthus and walnuts have complex leaves. Compound leaves are pinnately and palmately compound. In the first case, the leaves are located in two opposite rows along a common axis, as, for example, in white acacia and walnut, and in the second, they move away from one point, as, say, in horse chestnut or clover.

Leaf sizes vary widely among taxa and even within the same plant species. They can reach a length of 20 m, for example in a palm tree Raphia ruffia, native to tropical Africa and Madagascar. Vegetable asparagus has very small leaves ( Asparagus officinalis var. altilis), casuarina horsetail ( Casuarina equisetifolia) and tamarisk, or comb ( Tamarix spp.).

In most cases, the leaves are wide and flat, but sometimes they are cylindrical, like onions, needle-shaped, like pine trees, or scale-like, like cypress trees. There are leaves linear (in cereals), rounded (in nasturtium), ovoid (in the frame), heart-shaped (in linden), lanceolate (in willow), etc. Sometimes the so-called heterophyly (“multiple leaves”) – leaves of different shapes are formed on the same plant; for example, sassafras has five variants.

Leaves with smooth edges are called entire leaves. Among trees, such leaves can be seen, for example, in dogwood, lilac, rhododendron, eucalyptus, imbricated oak, loosestrife and virginiana. In many cases, the edges of the leaf blade are lobed, dissected, serrated, and notched. For example, red oak leaves are pinnately lobed with spiny protrusions of veins at the tops of the lobes, while white oak leaves are pinnately lobed or smoothly notched without sharp corners.

In most plants, the leaf arrangement is alternate, or spiral: the leaves, like the buds with lateral shoots, extend one at a time from each node, either on one or the other side of the stem. An example is all birches, elms, oaks and walnuts. In some species, in particular maples, viburnum and dogwood, leaves, buds and side shoots are located oppositely - on opposite sides of each node. When three or more leaves extend from a node, the leaf arrangement is called whorled. In any case, the leaves move away from the stem so as to minimally shade each other. They form a kind of “leaf mosaic” in space, designed to capture as much sunlight falling on the plant as possible.

Leaf blade.

A typical leaf blade consists of a thin layer of surface cells, the epidermis, and an underlying multilayered internal tissue, the mesophyll. The mesophyll is penetrated by a system of veins. A thin section of a leaf under a microscope shows that the outside of the epidermis is covered with a cuticle - a film consisting of a waxy cutin. This film is interrupted in places by inclusions of pectin-like substances. Through such areas, the leaf can absorb substances containing nitrogen, phosphorus, potassium and other elements necessary for the nutrition and normal functioning of the plant from solutions falling on its surface. The cuticle and epidermis protect the inner cells from rapid drying out, and the thickness of these outer layers often indicates the species' adaptation to its environment. Thus, in pines and other narrow-leaved evergreens, the powerful cuticle very effectively slows down evaporation, especially in winter, when the frozen soil contains little water available for the roots.

The cuticle and epidermis are permeated with tiny holes - stomata, the number of which is unequal on both sides of the leaf. Each stomata is a gap between two bean-shaped guard cells, which, slightly changing their shape, open or close it. This regulates the rate of transpiration, i.e. loss of water by the plant. When the stomata are open, water vapor evaporates through them into the atmosphere and this ensures the upward movement of new portions of water with salts dissolved in it from the roots to the leaves and other parts of the shoots. Gas exchange between the plant and the environment also occurs through stomata. Guard cells react sensitively to the level of light: when it increases, the stomata open wider; when it gets dark, the stomatal fissure becomes narrower. Thus, stomatal gas exchange and transpiration are much more intense during the day than at night.

The epidermis of the leaf also contains specialized stomata - hydathodes, which secrete water in the form of droplets. This process is called guttation. Its intensity is maximum when a lot of water is absorbed and evaporation is slow. Contrary to popular belief, dew drops observed on the grass on a summer morning are the result of guttation, and not the condensation of atmospheric moisture.

The main part of the leaf is mesophyll. Directly under the upper (sometimes also under the lower) epidermis there are one or several cylindrical layers perpendicular to the surface of the so-called leaf. palisade cells - palisade parenchyma. Each of these cells contains numerous miniature bodies - chloroplasts, containing the green pigment chlorophyll, which captures solar energy and converts it into chemical energy. This process, called photosynthesis, produces sugars from carbon dioxide in the atmosphere and water from the soil. Under the palisade parenchyma there are large cells that make up the spongy parenchyma. Free spaces between them (intercellular spaces) facilitate the diffusion of gases inside the leaf. Spongy parenchyma contains fewer chloroplasts, and photosynthesis is not as intense here as in palisade parenchyma.

Veins penetrating the leaf, i.e. vascular-fibrous bundles that conduct water and nutrients are surrounded by a bundle sheath, or lining, of thin-walled, compactly located cells. The upper part of the vein consists of xylem, formed by vessels and tracheids, and the lower part of phloem, represented mainly by sieve tubes. Through the xylem, water with dissolved mineral salts moves from the roots to the leaf blades, and through the phloem, photosynthesis products - organic substances - are sent from the leaf to all organs of the plant.

There are two main types of leaf venation - reticulate, when the veins branch and connect to each other, and parallel, when they run parallel to each other. The first type is typical for dicotyledonous flowering plants - geranium, tomato, maple, oak, etc.; the second is for monocots, i.e. iris, lilies, cereals (for example, corn, bamboo, wheat), etc. There are various deviations from this pattern and transitional types of venation.

Photosynthesis.

The main function of the leaf is photosynthesis, during which sugars are formed from water and carbon dioxide due to solar energy. From these sugars, substances specific to them are formed in various plant organs, which are necessary, for example, for growth, lignification of cells, ripening of fruits and seeds, etc. Sugars are stored in reserve so that they can be used if necessary. Thus, the green leaf is an organ on which the provision of plants with organic substances entirely depends. To grow, plants need the same organic substances as animals (proteins, fats, carbohydrates, etc.), but only photosynthesis allows them to be obtained from inorganic compounds. All living beings that are not capable of photosynthesis depend directly or indirectly on green plants for their nutrition.

The process of photosynthesis is very complex, and here we will consider it only in the most general terms. Typically, carbon dioxide enters the leaf from the atmosphere through the stomata, spreads through the intercellular space, passes through the cell wall and is absorbed by the fluid filling the cells. The carbon dioxide that gets inside the chloroplasts and the water that is always present here enter into a series of reactions that produce various intermediate products, ultimately sugars, in particular the water-soluble sugar glucose and the product of its polymerization, starch. Further, proteins are formed from sugars through certain reactions with nitrogen and sulfur compounds (coming mainly from the soil). All other compounds necessary for the plant, such as cellulose, lignin, fats, oils, etc., are ultimately built from sugars.

Development and fall of leaves.

Leaves develop from areas of rapidly growing stem tissue - the meristem, located in the buds at the apex of the stem and in the shoot nodes. Before the buds open, not yet dissected leaf primordia are formed from the meristem in the form of a tubercle or roller on the growth cone - the so-called. leaf primordia. As the bud opens, their cells begin to rapidly divide, grow, and specialize until the leaf is fully formed. As the leaf develops, in its axil, i.e. At the top of the angle between the leaf and the section of the stem going up from it, a new bud is almost always formed. New shoots may arise from such axillary buds next year.

Unlike roots and stems, a leaf is a temporary organ. Having reached full development, after some time it dies and falls off. In deciduous species in the temperate climate zone, this occurs every autumn. Before this, the plant hormone abscisin II stimulates the formation at the base of the leaf petiole (or its blade, if the leaf is sessile) of a special layer of specialized tissue, the so-called. separating layer. It consists mainly of spongy parenchyma, i.e. thin-walled cells loosely connected to each other, therefore, under the influence of its own weight, as well as external influences, such a leaf relatively easily breaks off from the stem. In evergreen species, the foliage is also renewed, but each leaf lives for several years, and the leaves do not fall all at once, but one by one, so that outwardly these changes are invisible. This phenomenon is widespread in tropical plants, on which at any time of the year you can see leaves in different stages of development: some are ready to fall, others are just unfolding, and others are passing the peak of maturity and metabolic activity.

Autumn leaf color.

The leaves become especially brightly colored in the fall in some geographic regions, for example, the northeastern and northwestern United States, southeastern continental Asia, and southwestern Europe. In Northern Europe, where winters are mild and rainy, the leaves turn mostly dirty yellow and brownish before falling.

Fall leaf color depends largely on the type of plant, but is also affected by weather conditions and soil type. Before the leaves fall, nutrients are transferred from them to the stems and roots. The formation of chlorophyll stops, and its remains are quickly destroyed by sunlight. As a result, yellow pigments, mainly xanthophylls and carotenes, become visible. They are present in the leaves throughout the growing season, but are masked by green chlorophyll in spring and summer.

The orange, red and purple tones of autumn foliage are caused by other pigments - anthocyanins, which, unlike yellow pigments, appear only in the fall, and their amount depends on the weather. If the air temperature drops sharply to a level of 0–7 ° C, more sugars and tannins remain in the leaf, and as a result, anthocyanin synthesis is activated.

Thus, if autumn is sunny, dry and cool, the leaves of many trees delight the eye with bright red, yellow, orange and crimson tones. If the autumn is cloudy and the nights are warm, then less sugar is synthesized in the leaves, and a significant proportion of it passes from them to the stem; under these conditions, the formation of anthocyanins is weak and the color of the foliage becomes predominantly dull yellow.

One of the most beautiful species in autumn is the sugar maple ( Acer saccharum), the leaves of which turn dark yellow, golden orange and bright red. At the red maple ( A. rubrum) they turn red, and in Norway maples ( A. platanoides) and silver ( A. saccharinum) acquire a golden yellow color. The autumn crown of the liquidambara resin-bearing tree, or amber tree, cannot but arouse admiration ( Liquidambar styraciflua): on the same tree it can shimmer in different shades of purple, scarlet, yellow and green. Among the shrubs, the winged euonymus ( Euonymus alatus), various types of barberry ( Berberis spp.) and American mackerel ( Cotinus americanus).

Specialized leaves.

Leaves can specialize in various ways, losing their typical appearance, structure and even functions. Examples of this are the tendrils of many legumes, which allow plants to cling to supports, the spines of cacti, in which the processes of photosynthesis have moved to green fleshy stems, the protective bud scales of trees, as well as bracts - scale-like covering leaves on the pedicels of many species. Sometimes the leaves surrounding flowers and entire inflorescences are bright and conspicuous, such as the white or red spathes of the aronica (calla, anthurium) or the red, white and pink apical leaves of poinsettia ( Euphorbia pulcherrima). They can easily be mistaken for petals, while the real flowers of these species can be relatively small and inconspicuous.

Agave americana ( Agave americana) the leaves are very thick and fleshy - they store water and nutrients. Other plants with a pronounced storage function of leaves include various purslanes (genus Portulaca) and sedums (genus Sedum). Their leaves contain mucilaginous colloidal substances that effectively bind water and slow down its evaporation in the arid habitat typical of these “leaf succulents.”

At the so-called Insectivorous plant leaves are turned into traps for small arthropods. So, in the Venus flytrap ( Dionaea muscipula) leaf halves, covered at the edges with spines protruding upward, can rotate relative to the midrib. When an insect lands on a leaf blade, these halves slam shut like a book, and the victim finds itself in a trap. Its body decomposes under the action of enzymes secreted by leaf glands, and the decomposition products are absorbed by the plant. At the pitcher's ( Nepenthes) there are modified leaves in the form of a pitcher. An insect that crawls in cannot get out; it drowns and is digested in the liquid secreted by the glands at the bottom of the jug.

The leaves of many plants are dangerous for warm-blooded animals. So, in rooting sumac ( Toxicodendron radicans) they contain an oily substance, which, when it gets on the skin, causes severe inflammation (dermatitis). In the leaves of some species of astragalus (genus Astragalus) selenium, which is toxic to animals, accumulates. Cattle that have eaten a large amount of these leaves become ill with selenosis, from which they sometimes die. Leaves are poisonous, for example, in plants such as Dieffenbachia painted ( Dieffenbachia picta), lily of the valley ( Convallaria majalis), azaleas and rhododendrons (genus Rhododendron), Kalmia latifolia ( Kalmia latifolia).