Leaf is a lateral vegetative organ of the shoot. It plays an important role in the life of the whole plant, the leaf structure is arranged in such a way that it is able to adapt to environmental conditions to perform its functions - photosynthesis, evaporation and gas exchange, guttation. The leaf can be modified and be a needle (as in conifers) or a thorn (in cacti and barberry, etc.). Such transformations of the lateral organs of the shoot help plants survive in various climatic zones.
The external structure of the leaf depends on the type of plant. So, they distinguish between simple and complex, petiolate, sessile and wrapping leaves. Almost all lateral organs of the shoot have an expanded part - a leaf blade, which can be whole, dissected, lobed or separate. The petiole, by which the main assimilating organ is attached to the stem, may be completely absent, then they say that the leaf is “sessile” or petiolate. If sheetthe plate completely encircles the stem, then it wraps around the lateral organ of the shoot. Petiole angiosperms also have stipules that protect young leaves and axillary buds.
The morphological structure of the leaf also proves the presence of simple and complex forms. The main assimilating organ of a plant is called simple if it has one petiole and one leaf blade, which fall off entirely (maple, lilac, willow). Compound leaves have 1 petiole and several leaf blades that can fall off individually (walnut, chestnut, ash).
The internal structure of the leaf is identical in all plants. The leaf blade is covered above and below with a layer of epidermis, which forms the skin. Some representatives of the flora on the upper skin may have hairs, a cuticle film, or a waxy coating. These are all protective devices that prevent overheating, burns, excessive evaporation of water. The integumentary tissue of most plants, on the underside of the leaf, has slit-like openings - stomata, which have two locking cells. Gases and water vapor pass through the stomatal apparatus, both into the lateral organ of the shoot and out.
The cellular structure of the leaf indicates the presence of the main tissue - the mesophyll, which is divided into spongy and palisade (columnar) parenchyma. Structural units of columnar tissue contain a huge number of chloroplasts that are able to move with sunlight. Cells are very close to each other, it is in them that photosynthesis takes place. sponge tissueIt is formed by elementary particles of the living, which have an irregular shape, a large amount of intercellular substance and are themselves very loosely laid.
Takes part, but not as actively as the palisade parenchyma, in assimilation, as well as through its air spaces, gas exchange occurs. Also in the leaf there are veins that act as vessels, participating in metabolism. It is through them that water with minerals enters the cells of the lateral organ of the shoot, and removes organic compounds formed during photosynthesis from the leaf itself. Also, large veins are surrounded by fibrous bundles formed by mechanical tissue and giving strength to the leaf.
Thus, the structure of the leaf is very complex and is determined by the functions that this organ performs - assimilation, gas exchange, guttation and evaporation. Also, in addition to the main ones, the leaf can perform additional functions - protection (thorns), a supply of substances (bulb scales) and vegetative reproduction.
