Sheet structure, external and internal
The leaf is the lateral vegetative organ of the shoot. It plays an important role in the life of the entire plant, the structure of the sheet is arranged so that it is able to adapt to environmental conditions to perform its functions - photosynthesis, evaporation and gas exchange, guttation. The sheet can be modified and represent a needle (like a coniferous tree) or a thorn (for cacti and barberry, etc.). Such transformations of the side branches of the shoot help plants survive in different climatic zones.
The external structure of the leaf depends on the plant species. So, distinguish simple and complex, petiolate, sessile and wrapping leaves. Almost all lateral organs of the shoot have an enlarged part - a leaf blade, which can be whole, dissected, lobed or separate. The petiole, which the main assimilating organ is attached to the stem, may be completely absent, then it is said that the leaf is "sedentary" or bessheretish. If the leaf blade completely encircles the stem, then it is a twisting side organ of the shoot. Angiosperms with petiolate leaves have stipules that protect young leaves and axillary buds.
Morphological structure of the leaf also provesthe presence of simple and complex forms. The main assimilating organ of a plant is called simple if it has one petiole and one leaf plate, which fall completely (maple, lilac, willow). Complex leaves have 1 petiole and several leaf blades that can fall separately (walnut, chestnut, ash).
The internal structure of the sheet is identical for allplants. The leaf plate on top and bottom is covered with a layer of epidermis, which forms the skin. Some representatives of the flora on the upper skin may have hairs, cuticle film or a wax coating. These are all protective devices that prevent overheating, burns, excessive evaporation of water. The integumentary tissue in most plants, on the underside of the leaf, has slotted holes - stomata, which have two closing cells. Through the stomatal apparatus pass gases and water vapor, both into the lateral organ of the shoot itself, and outward.
Cellular structure of the leaf, indicating the presence ofthe main tissue - mesophyll, which is divided into spongy and palisade (columnar) parenchyma. Structural units of columnar tissue contain a huge amount of chloroplasts, which are capable of moving behind sunlight. Cells are very closely adjacent to each other, it is in them is photosynthesis. The spongy tissue is formed by elementary living particles, which have an irregular shape, a large amount of intercellular substance, and are themselves very loosely laid.Participates, but not as active aspalisade parenchyma, in assimilation, as well as through its airspace, gas exchange takes place. Also in the leaf are veins, which serve as vessels, participating in the metabolism. It is through them that water with mineral substances enters the cells of the side organ of the shoot, and from the leaf itself displays organic compounds formed during photosynthesis. Also large veins are surrounded by fibrous bundles formed by a mechanical tissue and imparting strength to the leaf.
Thus, the structure of the sheet is very complex andis due to the functions performed by this body - assimilation, gas exchange, gutation and evaporation. Also, in addition to the basic, the sheet can perform additional functions - protection (spines), a stock of substances (scales of bulbs) and vegetative reproduction.