The main part of an ordinary sheet is its plate. leaf blade- This is an expanded flat formation that performs the functions of photosynthesis, gas and water exchange. In addition to the lamina, the leaves often have petiole- an elongated cylindrical stem-like part, with the help of which the plate is attached to the stem. If there is a petiole, the leaf is called petiolate, and if it is absent, it is called sessile. The bottom of the sheet is base- can grow and cover the stem in the form of a tube. This formation is called a leaf sheath. Quite often, at the base of the leaf, there are special outgrowths at the petiole - stipules. Stipules are paired various shapes and sizes, green or colorless, free or fused with petiole. Stipules may or may not fall as the leaf grows.

Leaves are called simple if they have one leaf blade on the petiole, and in a complex leaf, several plates, called leaflets, are attached to one petiole.

Simple sheet. The leaf blade of a simple leaf can be whole or, on the contrary, dissected, i.e. in varying degrees, indented, consisting of protruding parts of the plate and notches. To determine the nature of dissection, the degree and shape of the indentation of leaf blades and the correct name of such leaves, first of all, it should be taken into account how the protruding parts of the blade are distributed - lobes, lobes, segments - in relation to the petiole and to the main vein of the leaf. If the protruding parts are symmetrical to the main vein, then such leaves are called pinnate. If the protruding parts come out as if from one point, the leaves are called palmate. By depth of cuts leaf blade leaves are distinguished: lobed, if the recesses (the depth of the cuts) do not reach half the width of the half-plate (the protruding parts are called lobes); separate, with a depth of cuts that go deeper than half the width of the half-plate (protruding parts - lobes); dissected, with a depth of incisions reaching the main vein or almost touching it (protruding parts - segments).

Complex sheet. Compound leaves, by analogy with simple ones, are called pinnate and palmate with the addition of the word "complex". For example, pinnate, palmate, ternary, etc. If a compound leaf ends with one leaflet, the leaf is called odd-pinnate. If it ends with a pair of leaflets, then it is called paro-pinnate.
The dismemberment of the plate of a simple leaf, as well as the branching of parts of a complex leaf, can be multiple. In these cases, taking into account the order of branching or dismemberment, they speak of double-, thrice-, four-pinnate or palmate, simple or complex leaves.

The main forms of the leaf blade

Types of division of simple leaf blades and classification of compound leaves

Main types of sheet edge

1 - whole; 2 - notched; 3 - wavy; 4 - prickly; 5 - gear; 6 - double-toothed; 7 - serrated; 8 - gorodchaty

Top shapes The shape of the top, base and edge of the leaf blades are also features used in the description and definition of plants.

The main forms of the top of the leaf blade

1 - spinous; 2 - pointed; 3 - pointed, or sharp; 4 - blunted; 5 - rounded; 6 - truncated; 7 - notched

Forms of the base of the leaf blade

1 - heart-shaped; 2 - kidney-shaped; 3 - swept; 4 - spear-shaped; 5 - notched; 6 - round; 7 - round-wedge-shaped; 8 - wedge-shaped; 9 - drawn; 10 - truncated

Main types of leaves

1 - needle-shaped (needles); 2 - linear; 3 - oblong; 4 - lanceolate; 5 - oval; 6 - elliptical, arcuate, entire; 7 - rounded; 8 - ovoid, peritoneal, dentate; 9 - obovate; 10 - rhombic; 11 - spatulate; 12 - heart-shaped, crenate; 13 - kidney-shaped; 14 - swept; 15 - spear-shaped; 16 - pinnate; 17 - palmate-lobed, finger-nervous; 18, 19 - finger dissected; 20 - lyre-shaped; 21 - ternary; 22 - palmate; 23 - paired pinnate, with stipules and antennae; 24 - unpaired pinnate with stipules; 25 - doubly pinnate; 26 - multiple pinnate; 27 - discontinuous pinnate; 28 - scaly

Isn't it paradoxical that, speaking about the world around us, we, without thinking about it, perceive it as green?
This is easily explained: as long as there are green plants, creating with the help of light from carbon dioxide organic matter - the basis of the life of all the rest - we also live ...

But why are plants green?
All objects we see only due to the fact that they reflect the rays of light falling on them. For example, a sheet of clean paper, perceived by us as white, reflects all parts of the spectrum. And an object that seems black to us absorbs all the rays. It is easy to understand that if the fibers of the fabric are impregnated with a substance that absorbs all the rays of light, except for red ones, then we will perceive the dress sewn from this fabric as red.
Similarly, chlorophyll - the main plant pigment - absorbs all rays except green ones. And it does not just absorb, but uses their energy in its own interests, especially actively - the red part of the spectrum, opposite to the green one.

And yet the leaves of plants are not always green. That is what will be the subject of my story. Of course, I will state many things in a very simplified way (may the professionals forgive me). But it seems to me that every person who is seriously involved in their cultivation should have an idea of ​​\u200b\u200bthe reasons for the change in the color of the leaves of plants.

Non-green greens

Several pigments are constantly present in the tissues of any living plant. Of course, the main one is green - chlorophyll, which determines the basic color of the leaves.
But there is also anthocyanin, which actively absorbs green rays and completely reflects red ones.
Pigment xanthosine absorbs all rays except yellow, and carotene reflects a whole group of rays and seems to us orange-carrot.
There is also a pigment called betulin which stains plant tissue White color(but it is found only in birch; and then - not in the leaves, but in the bark, and therefore we will not talk about it).

We see all additional leaf pigments only after the death of chlorophyll. For example, on the leaves of plants with the arrival of autumn cold weather or as a result of leaf aging, as happens with popularly beloved codiaums.
Bright variegated leaves, being its only decoration, in fact, are dead and no longer give anything to the plant. Breeders only chose clones that can keep these useless but beautiful old leaves for as long as possible.

Probably, many flower growers had to observe the reddening of the leaves of plants exposed to excessively bright sunlight. In everyday life, this phenomenon is called "sunburn". But when we sunbathe, for protection from exposure ultraviolet radiation The skin produces a special pigment called melanin. In plants, no new pigments are produced, but on the contrary, chlorophyll is destroyed; then the anthocyanin previously present in the tissues becomes visible. It is clear that such reddening of the leaves is an alarm for the plant owner.

By the way, the leaves of some plants (y - stems) with an excess of light sometimes acquire a bluish color. This is due to the development of a wax layer on the surface of the fabric, which very effectively reflects all the rays of light, but especially actively - blue and blue.

It is very interesting to solve the problem of maximizing the use of light by plants living in conditions of its constant deficiency. For example, under the canopy of a tropical forest.
Many paid attention to the leaves, in which the upper surface of the leaf is dark green, and the lower is deep red. It is clear that in this case we are not talking about the destruction of chlorophyll.
The fact is that the rays of light when passing through a thin sheet plate are far from completely absorbed: part of the light passes through the leaf and is lost by the plant. It is this problem that the lower surface of the leaf stained with anthocyanin solves. It reflects especially valuable red rays back into the leaf, i.e. causes them to re-pass through the chloroplasts. It is clear that the efficiency of the use of light rays in such a sheet increases significantly.

An important function of additional plant leaf pigments is to capture photons in the yellow-green part of the spectrum, which is not used by chlorophyll. As a result, the overall efficiency of photosynthesis increases.
I will give as an example passionflower three-lane(Passiflora trifasciata). Among the huge variety, this species stands out. Perhaps this is the only passionflower grown solely for the sake of decorative leaves. Their red-violet color, which changes depending on the illumination, is due to the presence of additional pigments that actively use all parts of the incident light spectrum. In addition, a silver stripe runs through the center of each leaf blade. In general, the color of the leaves of this passionflower resembles the elegant coloring of the leaves of royal begonias.

However, in bright light, the leaves of passionflower with a three-stripe become simply green, and at best, separate silver specks remain from the stripes. The fact is that the silvery stripes are nothing more than a cluster of air-filled cells that equally refract all the rays of light passing through them. Some of them are reflected, and therefore we perceive them as silver-white, and most of them are directed inside the sheet plate. In other words, these hollow cells act like lenses, greatly increasing the efficiency of photosynthesis. It is clear that in plants with sufficient illumination, the need for this adaptation of the leaves disappears, and then the hollow cells are filled with chlorophyll.

The program that directs the plant to produce chlorophyll is written down at the gene level. More than a hundred genes are known to be involved in this process. But this complex mechanism sometimes fails - plants appear in which either part of the leaf plate or individual leaves are completely devoid of chlorophyll. Then the cells of the leaf can be filled with additional pigments (in this case, the leaf acquires the appropriate color) or simply become hollow, and therefore appear white.

Of course, from the point of view of healthy physiology, such plants must be considered inferior. But in practical floriculture, they are especially decorative, they are readily grown.

When dealing with such plants, it should be borne in mind that they are much more capricious than their green counterparts and therefore are especially demanding on. After all, the lack of chlorophyll in the leaves in the first place entails a decrease in plant nutrition. Therefore, with insufficient lighting, their leaves quickly lose their former brightness and variegation of color, become faded and oppressed.

In addition, lovers of such plants should remember that excess nitrogen in the soil can lead to the disappearance of leaf spot due to the accumulation of chlorophyll.
And one more thing: during the reproduction of such plants, the inheritance of the variegated color of the leaves is possible only in cuttings. Seedlings (and sometimes leaf cuttings) turn into normally colored, green specimens.

tricky leaves

Special mention should be made of the unusual leaves of some members of the Mesembryanthemum (Aizoon) family, and first of all, Lithops.

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The leaf is a vegetative organ of plants, is part of the 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 conditions of existence, leaves, changing, can serve the following purposes.

• Accumulation of nutrients (onion, cabbage), water (aloe);
• protection against being eaten by animals (thorns of cactus and barberry);
• vegetative propagation (begonia, violet);
• catching and digesting insects (dew, venus flytrap);
• movement and strengthening of a weak stem (pea tendrils, wikis);
• removal of metabolic products during leaf fall (in trees and shrubs).

General characteristics of a plant leaf

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

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

• leaf blade;
• petiole, with which the leaf is attached to the stem;
• stipules.

Some plants do not have petioles, such leaves, unlike petioles, are called sedentary. Stipules are also not found in all plants. They represent various sizes paired appendages at the base of the leaf petiole. Their form is diverse (films, scales, small leaves, spines), their function is protective.

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

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

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

The edges of the leaf blade can be solid, such a sheet is called whole-edge (lilac) or notched. Depending on the shape of the notch, along the edge of the leaf blade, there are serrate, serrate, crenate, etc. In serrated leaves, the serrations have more or less equal sides (beech, hazel), in serrate - one side of the tooth is longer than the other (pear), crenate - have sharp notches and blunt bulges (sage, budra). All these leaves are called whole, since their recesses are shallow, do not reach the width of the plate.

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

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

The lower, or first, leaves of the shoot are the scales of the kidneys, the outer dry scales of the bulbs, the cotyledon leaves. The lower leaves usually fall off during the development of the shoot. The leaves of the basal rosettes also belong to the grassroots. Median, or stem, leaves are typical for plants of all kinds. 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).

Sheet arrangement types

There are three main types of leaf arrangement:

• Regular or spiral;
• opposite;
• whorled.

At the next arrangement, single leaves are attached to the stem nodes in a spiral (apple, ficus). With the opposite - two leaves in the node are located one against the other (lilac, maple). Whorled leaf arrangement - three or more leaves in a node cover the stem with 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 obscure 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 cells of the skin contain the cytoplasm and nucleus, are located in one continuous layer. Their outer shells are thickened.

Stomata are the respiratory organs of a plant.

In the skin are stomata - gaps formed by two trailing, 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, facing the gap, is thicker than the opposite.

A change in the turgor of the guard cells changes their shape, due to which the stomatal opening is open, narrowed or completely closed, depending on the conditions. environment. So, during the day, the stomata are open, and 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 there are also on the upper, 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 species and growth conditions. On average, there are 100-300 of them per 1 mm 2 of the surface, but there can be much more.

Leaf pulp (mesophile)

Between the upper and lower skin of the leaf blade is the pulp of the leaf (mesophile). Under top layer there is 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 are carried out.

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

The flesh of the leaf is permeated with a dense network of veins, vascular-fibrous bundles that supply the leaf with water and substances dissolved in it, as well as the removal of 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 them to the top, they become thinner due to branching and 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 conditions growth, first of all, from the conditions of lighting and water supply. Plants in shaded places often lack palisade perenchyma. The cells of the assimilation tissue have larger palisades, the concentration of chlorophyll in them is higher than in photophilous plants.

Photosynthesis

In the chloroplasts of the pulp cells (especially the columnar parenchyma), the process of photosynthesis takes place 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%).

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

Evaporation of water from 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 stomata play the main role in evaporation, and the entire surface of the leaf also partially takes part in this process. In this regard, stomatal transpiration and cuticular transpiration are distinguished - through the surface of the cuticle covering the leaf epidermis. Cuticular transpiration is much less than stomatal: in old leaves, 5-10% of total transpiration, but in young leaves with a thin cuticle, it can reach 40-70%.

Since transpiration is carried out mainly through the stomata, where carbon dioxide also enters 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 a plant evaporates to build 1g of dry matter is called transpiration coefficient. Its value ranges from 30 to 1000 and depends on the growth conditions, type and variety of plants.

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

Transpiration creates a suction force in the cell of the leaf and root, 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 life processes of the plant, especially photosynthesis.

Plants in arid places, as well as in dry weather, evaporate more water than in conditions high humidity. Evaporation of water, except for stomata, is regulated by protective formations on the skin of the leaf. These formations are: cuticle, wax coating, pubescence from various hairs, etc. In succulent plants, the leaf turns into spines (cacti), and the stem performs its functions. Plants of wet habitats have large leaf blades, there are no protective formations on the skin.

Transpiration is the mechanism by which water is evaporated from the leaves of a plant.

With difficult evaporation in plants, guttation- the release of water through the stomata in a drop-liquid state. This phenomenon occurs in nature usually in the morning, when the air approaches saturation with water vapor, or before rain. Under laboratory conditions, guttation can be observed by covering young wheat seedlings with glass caps. Across short term droplets of liquid appear on the tips of their leaves.

Isolation system - leaf fall (leaf fall)

The biological adaptation of plants to protection from evaporation is leaf fall - a massive fall of leaves in the cold or hot season. In temperate zones, trees shed their leaves for the winter when the roots cannot supply water from the frozen soil and frost dries out the plant. In the tropics, leaf fall is observed during the dry season.

Preparation for shedding leaves begins with a weakening of the intensity of life processes 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, parenchymal cells begin to divide and form a separating layer. After that, the leaf comes off, and a trace remains on the stem - a leaf scar. By the time of leaf fall, the leaves are aging, unnecessary metabolic products accumulate in them, which are removed from the plant along with the fallen leaves.

All plants (usually trees and shrubs, less commonly herbs) are divided into deciduous and evergreen. In deciduous leaves develop during one growing season. Every year with the onset adverse conditions they fall off. Leaves of evergreen plants live from 1 to 15 years. The death of part of the old and the appearance of new leaves occurs constantly, the tree seems evergreen (coniferous, citrus).

Sheet - This is a lateral specialized part of the shoot.

Basic and advanced sheet functions

Main: functions of photosynthesis, gas exchange and water evaporation (transpiration).

Additional: vegetative propagation, storage of substances, protective (thorns), supporting (antennae), nutritious (at carnivorous plants), removal of some metabolic products (with falling leaves). Leaves grow mainly to certain sizes due to regional meristems . Their growth is limited (unlike the stem and root) only to a certain size. The sizes are different, from a few millimeters to several meters (10 or more).

Life span is different. In annual plants, the leaves die off along with other parts of the body. perennial plants can replace foliage gradually, during the growing season or throughout life - evergreen plants (laurel, ficus, monstera, lingonberry, heather, periwinkle, cherry laurel, palm tree, etc.). Falling leaves during unfavorable seasons is called - leaf fall . Plants that show leaf fall are called deciduous (apple, maple, poplar, etc.).

The sheet consists of leaf blade And petiole . The leaf blade is flat. On the leaf blade, you can select the base, tip and edges. In the lower part of the petiole there is a thickened base sheet. Branches in the leaf blade veins - vascular fibrous bundles. Separate the central and lateral veins. The petiole rotates the plate to better capture the rays of light. The leaf falls off along with the petiole. Leaves with a petiole are called petiolate . Petioles are short or long. Leaves without a petiole are called sedentary (eg, corn, wheat, foxglove). If Bottom part leaf blade covers the stem in the form of a tube or groove, then a leaf is formed vagina (in some grasses, sedges, umbrella plants). It protects the stem from damage. The shoot can penetrate the leaf blade through and through - pierced leaf .

petiole shapes

On the cross section, the petioles can be shaped: cylindrical, ribbed, flat, winged, grooved, etc.

Some plants (rosaceae, legumes, etc.), in addition to the blade and petiole, have special outgrowths - stipules . They cover the lateral kidneys and protect them from damage. Stipules can look like small leaves, films, spines, scales. In some cases, they are very large and play an important role in photosynthesis. They are free or attached to the petiole.

The veins connect the leaf to the stem. These are vascular fibrous bundles. Their functions: conductive and mechanical (veins serve as a support, protect leaves from tearing). The location, branching of the veins of the leaf blade is called venation . There are venation from one main vein, from which lateral branches diverge - net, pinnate (bird cherry, etc.), palmate (Tatar maple, etc.), or with several main veins that run almost parallel to one another -– arc (plantain, lily of the valley) and parallel (wheat, rye) venation. In addition, there are many transitional types of venation.

Most dicots are characterized by pinnate, palmate, reticulate venation, while monocots have parallel and arcuate venation.

Leaves with straight veins are mostly entire.

Variety of leaves by external structure

By leaf blade:

Distinguish between simple and compound leaves.

simple leaves

Simple leaves have one leaf blade with a petiole, which can be complete or dissected. Simple leaves fall completely during leaf fall. They are divided into leaves with a whole and dissected leaf blade. Leaves with an entire leaf blade are called whole .

The forms of the leaf blade differ in the general contour, the shape of the apex and base. The contour of the leaf blade can be oval (acacia), heart-shaped (linden), needle-shaped (coniferous), ovoid (pear), swept (arrowhead), etc.

The tip (apex) of the leaf blade can be sharp, blunt, blunt, pointed, notched, cirriform, etc.

The base of the leaf blade can be round, heart-shaped, swept, spear-shaped, wedge-shaped, uneven, etc.

The edge of the leaf blade can be entire or notched (do not reach the width of the blade). According to the shape of the grooves along the edge of the leaf blade, the leaves are serrated (the teeth have equal sides - hazel, beech, etc.), serrate (one side of the tooth is longer than the other - pear), pubescent (the grooves are sharp, the bulges are blunt - sage), etc.

compound leaves

Complex leaves have a common petiole (rachis). Simple leaves are attached to it. Each of the leaves can fall off on their own. Compound leaves are divided into trifoliate, palmate and pinnate. Complex trifoliate leaves (clover) have three leaflets, which are attached to a common petiole with short petioles. Palmately complex the leaves are similar in structure to the previous ones, but the number of leaflets is more than three. Pinnate leaves consist of leaflets located along the entire length of the rachis. There are paired pinnate and unpaired pinnate. Paranopinnate leaves (sowing peas) consist of simple leaflets, which are arranged in pairs on the petiole. Unpaired pinnate leaves (rosehip, mountain ash) end with one unpaired leaflet.

According to the method of division

Leaves are divided into:

1) bladed if the articulation of the leaf blade reaches 1/3 of its entire surface; protruding parts are called blades ;

2) separate if the segmentation of the leaf blade reaches 2/3 of its entire surface; protruding parts are called shares ;

3) dissected , if the degree of articulation reaches the central vein; protruding parts are called segments .

leaf arrangement

This is the arrangement of leaves on a stem in a certain order. The leaf arrangement is a hereditary trait, but during the development of the plant, when adapted to lighting conditions, it can change (for example, in the lower part the leaf arrangement is opposite, in the upper part it is next). There are three types of leaf arrangement: spiral, or alternate, opposite and annular.

Spiral

Inherent in most plants (apple, birch, wild rose, wheat). In this case, only one leaf departs from the node. The leaves are arranged in a spiral on the stem.

Opposite

In each node, two leaves sit one opposite the other (lilac, maple, mint, sage, nettle, viburnum, etc.). In most cases, the leaves of two adjacent pairs depart in two mutually opposite planes, without shading each other.

ringed

More than two leaves depart from the node (elodea, crow's eye, oleander, etc.).

The shape, size and arrangement of the leaves are adapted to the lighting conditions. The mutual arrangement of the leaves resembles a mosaic if you look at the plant from above in the direction of the light (hornbeam, elm, maple, etc.). This arrangement is called sheet mosaic . At the same time, the leaves do not obscure each other and use the light efficiently.

Outside, the leaf is covered mainly with a single-layered, sometimes multi-layered epidermis (skin). It consists of living cells, most of which lack chlorophyll. Through them, the sun's rays easily reach the lower layers of leaf cells. In most plants, the skin secretes and creates a thin film of fatty substances on the outside - the cuticle, which almost does not let water through. On the surface of some skin cells there may be hairs, spines that protect the leaf from damage, overheating, excessive evaporation of water. In plants that grow on land, on the underside of the leaf in the epidermis there are stomata (in wet places(cabbage) - stomata on both sides of the leaf; in water plants (water lily), the leaves of which float on the surface, on the upper side; Plants that are completely submerged in water do not have stomata. Stomatal functions: regulation of gas exchange and transpiration (evaporation of water by foliage). Average per 1 square millimeter surface has 100–300 stomata. The higher the leaf is located on the stem, the more stomata per unit surface.

Between the upper and outer layers of the epidermis are the cells of the main tissue - the assimilation parenchyma. In most angiosperm species, two types of cells of this tissue are distinguished: columnar (palisade) And spongy (loose) chlorophyll-bearing parenchyma. Together they make mesophyll sheet. Under the upper skin (sometimes also above the lower one) contains a columnar parenchyma, which consists of cells of the correct shape (prismatic), arranged vertically in several layers and tightly adjacent to one another. Loose parenchyma is located under the columnar and above the lower skin, consists of irregularly shaped cells that do not fit tightly to one another and have large intercellular spaces filled with air. Intercellular spaces occupy up to 25% of the leaf volume. They connect with stomata and provide gas exchange and leaf transpiration. It is believed that photosynthesis processes are more intense in the palisade parenchyma, since its cells have more chloroplasts. In the cells of the loose parenchyma, there are much fewer chloroplasts. They actively store starch and some other nutrients.

Through the tissues of the parenchyma pass vascular fibrous bundles (veins). They include conductive tissue - vessels (in the smallest veins - tracheids) and sieve tubes - and mechanical. Above the vascular fibrous bundle is the xylem, and below is the phloem. Organic substances that were formed during photosynthesis flow through sieve tubes to all plant organs. Through the vessels and tracheids, water with minerals dissolved in it enters the leaf. The mechanical tissue gives strength to the leaf blade, the support of the conductive tissue. Between the conducting system and the mesophyll is free space or apoplast .

Sheet modifications

Leaf modifications (metamorphoses) occur when additional functions are performed.

tendrils

They allow the plant (pea, vetch) to cling to objects and fix the stem in an upright position.

spines

Occur in plants that grow in dry places (cactus, barberry). In Robinia pseudoacacia (white locust), spines are modifications of stipules.

Scales

Dry scales (buds, bulbs, rhizomes) perform a protective function - they protect against damage. Fleshy scales (bulbs) store nutrients.

In insectivorous plants (dew), the leaves are modified to trap and digest mainly insects.

phyllodes

This is the transformation of the petiole into a leaf-shaped flat formation.

Leaf variability is due to a combination of external and internal factors. The presence of leaves of different shapes and sizes in the same plant is called heterophilia , or diversity . It is observed, for example, in water yellow, arrowhead, etc.

(from lat. trans - through and spiro - I breathe). This is the removal of water vapor by the plant (evaporation of water). Plants absorb a lot of water, but use only a small part of it. All parts of the plant evaporate water, but especially the leaves. Evaporation creates a special microclimate around the plant.

Types of transpiration

There are two types of transpiration: cuticular and stomatal.

Cuticular transpiration

Cuticular transpiration is the evaporation of water from the entire surface of the plant.

Stomatal transpiration

stomatal transpiration is the evaporation of water through the stomata. The most intense is stomatal. Stomata regulate the rate of water evaporation. The number of stomata different types plants are different.

Transpiration promotes the flow of a new amount of water to the root, raising water along the stem to the leaves (with the help of suction force). In this way root system forms the lower water pump, and the leaves form the upper water pump.

One of the factors determining the rate of evaporation is air humidity: the higher it is, the less evaporation (evaporation stops when the air is saturated with water vapor).

The value of water evaporation: reduces the temperature of the plant and protects it from overheating, provides an upward flow of substances from the root to the aerial part of the plant. The intensity of photosynthesis depends on the intensity of transpiration, since both of these processes are regulated by the stomatal apparatus.

This is the simultaneous dropping of leaves for a period of adverse conditions. The main causes of leaf fall are a change in the length of daylight hours, a decrease in temperature. This increases the outflow of organic matter from the leaf to the stem and root. It is observed in autumn (sometimes, in dry years, in summer). Leaf fall is a plant adaptation to protect against excessive water loss. Together with the leaves, various harmful products metabolism, which are deposited in them (for example, calcium oxalate crystals).

Preparation for leaf fall begins even before the onset of an unfavorable period. A decrease in air temperature leads to the destruction of chlorophyll. Other pigments become visible (carotenes, xanthophylls), so the leaves change color.

Petiole cells near the stem begin to intensively divide and form across it separating a layer of parenchyma that is easily exfoliated. They become rounded and smooth. Large intercellular spaces arise between them, which allow cells to easily separate. The leaf remains attached to the stem only thanks to the vascular-fibrous bundles. On the surface of the future leaf scar preformed protective layer cork fabric.

Monocots and herbaceous dicots do not form a separating layer. The leaf dies, gradually collapses, remaining on the stem.

Fallen leaves are decomposed by soil microorganisms, fungi, and animals.

Autumn is one of the most beautiful times of the year. The diversity and richness of nature during this period simply amaze the mind, the simple and complex leaves are so different from each other. The leaf arrangement of each plant is special (it can be alternate or whorled), and it is from it that you can determine which species it belongs to. Let's take a closer look at the features and functions of each type of leaf.

Definition in botany

Along with flowers, roots, stems and shoots, leaves are the most important vegetative organs in plants, which are also responsible for the function of photosynthesis. In addition, they perform many other tasks, for example, they are involved in the processes of respiration, evaporation and guttation of plants. There are the following simple and complex, each of them has its own characteristics and is found in a certain type of plant.

Very often, leaf blades are mistaken for leaves, but in fact it is an organ that consists of a blade (veins pass through it) and a cutting that originates at the base and connects the leaf blade with stipules. It always occupies a lateral position on the stem, and all the leaves are arranged on it in a certain sequence in such a way as to provide optimal access to the sun's rays. Its size can vary from 2 cm to 20 m (for tropical palms).

External structure and forms

One of the features of these organs is their flat shape, which ensures maximum contact of the plant surface with the air and sunlight. The forms are simple and differ from each other in appearance. Simple ones have only one leaf blade, which is connected to the base with the help of a petiole. Complex ones consist of several leaf blades located on one petiole. Remember what the thickest vein looks like in the middle, to which two or three stipules are attached on each side. Such a complex is called opposite, because the leaf blades are located symmetrically to each other.

The main components are the plates and veins that run along their surface, as well as the petiole, stipules (although not all plants have them) and the base, with which the element is connected to the stem of a tree or other plant.

Unlike the shape of a simple sheet, complex ones can have several varieties that have their own distinctive properties and features.

Internal structure

The upper surface of the leaf blades is always covered with a skin, which consists of a layer of colorless cells of the integumentary tissue - the epidermis. The main functions of the skin are protection from external mechanical damage and heat transfer. Due to the fact that its cells are transparent, sunlight passes freely through it.

The undersurface is also composed of these transparent cells, closely adjacent to each other. However, among them there are small paired green cells, between which there is a gap. It is this part that is called the stoma. Opening and connecting again, green cells open and close the entrance to the stomata. During these movements, the evaporation of moisture and the process of gas exchange occur. It is known that from 90 to 300 stomata per 1 mm 2 fall on the surface of one leaf plate.

Interesting fact: green cells are almost always located on the side of the leaf on which the maximum air exchange occurs. So, for example, in plants floating on water, capsules or water lilies, the stomata are on the outside, facing the air.

Varieties

Scientists distinguish two main types of leaves: this leaf is simple and complex. The structure of each of them has its own characteristics. Depending on the appearance, the number of plates and the shape of their edges, compound leaves can also be divided into several types. So, here are the most common types, if selected by external signs:

• fan-shaped (the shape resembles a semicircle);
• spear-shaped (sharp, sometimes there are spines on the surface);
• lanceolate (rather wide, with narrowed edges);
• oval (ovoid shape, which is slightly pointed closer to the base);
• palmate and lobed (they can sometimes be confused, since they both have several lobes);
• palmate (plates diverge from petiole, appearance resembles fingers)
• needle (thin and rather sharp).

This list can be continued for a long time, however, the complex shape of the leaf has several more types, depending on the shape of the edges, as well as the location of the leaf blades themselves.

Types of compound plants

Along the edges of the plates, it is very often possible to determine which species a particular plant belongs to. The following forms occur most frequently in nature:

• whole-edge - have smooth edges, on which there are no teeth;
• serrated - as the name implies, such leaves have teeth along the edges;
• fine-toothed - these resemble a saw, which has very sharp and small incisors;
• wavy - these have wavy cutouts that do not have a strict order or standard shape.

Features of each type

It is worth talking more about the distinguishing features of simple and compound leaves, as this can help determine what kind of plant it is and what species it belongs to. So, one of the most noticeable features of each species is the number of plates. If three elements are present, then we have triple-shaped sheets. If five are palmate, and if more, then they are called pinnately divided. On each plate, one can observe a special venation system, due to which nutrients enter the internal tissues. In simple and complex varieties, they differ in shape and structure. Here are the most common types of vein arrangement:

• arcuate (when the venation resembles a menorah in shape - one of the symbols of Judaism);
• transverse;
• longitudinal;
• palmate;
• parallel;
• mesh;
• pinnate.

Another distinguishing feature is the way the leaves are arranged on the stem. Simple and complex - all, without exception, are attached to plant stems in two ways:

• with the help of a cutting, in which case the plant belongs to petioles;
• without a cutting, when the base grows and covers the stem, then we have a sessile plant in front of us.

Leaves of plants: simple and complex

If we classify plants according to the characteristics of leaves, then the following facts can be noted. Simples are commonly found in all herbaceous plants, including shrubs and trees. Complex ones are found both in shrubs and in trees, however, unlike simple ones, during leaf fall they do not fall off all at once, but in parts: first the plates themselves, and then the stalk.

Let's look at examples of the name of simple and complex leaves in plants. In most trees growing in Russia, the leaves belong to plain sight. Aspen, birch and poplar have different shapes: lanceolate, rounded with jagged edges and spear-shaped, respectively. With the onset of autumn colds, the leaves of each of them crumble entirely. They are also found in fruit trees like apple, pear and cherry; agricultural crops such as oats and corn also have simple leaves.

Complex forms are present on leguminous plants, for example, pinnate leaves in peas. The following trees have palm-shaped leaves: maple, chestnut, lupine, etc. Remember red clover, its shape is called ternary with ciliated edges.

What are the functions of leaves?

Simple and complex forms of these organs are largely due to climatic conditions. In hot countries trees have leaves large sizes, which serve as a kind of protective fence from the sun's rays.

However, the main irreplaceable function is participation in photosynthesis. As you know, it is thanks to this process that trees can convert carbon dioxide into oxygen by absorbing solar energy.

The second most important process is cellular respiration. With the help of mitochondria, the leaves take in oxygen, and carbon dioxide is exhaled through the stomata, which is then used during photosynthesis. Since photosynthesis occurs only in the presence of light, at night carbon dioxide is stored in the form of organic acids.

Transpiration is the evaporation of water from the leaf surface. This regulates the overall temperature and humidity of the plant. The intensity of evaporation depends on the size and thickness of the plates and on the wind speed at a certain point in time.

Adaptation and modifications

Many leaves - simple and complex - have the ability to adapt to environmental conditions. In the process of evolution, they have acquired the ability to change. Here are the most amazing ones:

• the ability to produce wax that lays on the surface and prevents excessive evaporation of water droplets;
• form reservoirs for water during rains, this happens due to the fusion of the edges in such a way that a bag-shaped container is formed (such forms can be found in many tropical vines);
• the ability to change the surface of the plates, cut leaves prevent the impact strong winds thereby protecting plants from damage.

Many facts related to the vital activity of these irreplaceable plant organs are still poorly understood. These wonderful decorations of nature itself, in addition to the above functions, perform another aesthetic task - they delight people with their splendor and variety of bright colors!