Leaf is a lateral vegetative organ of the shoot. It plays an important role in the life of the whole plant, the structure of the leaf is arranged so that it is able to adapt to environmental conditions to perform its functions - photosynthesis, evaporation and gas exchange, gutting. The leaf can be modified and be a needle (like 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, distinguish between simple and complex, petiole, sessile and entwining leaves. Almost all the lateral organs of the shoot have an expanded part - the 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 it is said that the leaf is “sessile” or ungill. If the leaf blade completely encircles the stem, then this is the twisting side organ of the shoot. Angiosperms with petiole have stipules that protect young leaves and axillary buds.
The morphological structure of the leaf also proves the presence of simple and complex forms. Simple is called the main assimilating organ of the plant, if it has one petiole and one leaf blade, which fall entirely (maple, lilac, willow). Complex leaves have 1 petiole and several leaf blades that can fall individually (walnut, chestnut, ash).
The internal structure of the leaf is identical in all plants. The leaf blade at the top and bottom is covered with a layer of the epidermis, which forms the skin. Some representatives of the flora on the upper skin may have hairs, a cuticle film or a wax 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 shoot organ itself and to the outside.
The cellular structure of the leaf indicates the presence of the
main tissue - mesophyll, which is divided into spongy and palisade (columnar) parenchyma. The structural units of columnar tissue contain a huge number of chloroplasts that are able to move behind sunlight. Cells are very close to each other, it is in them that photosynthesis occurs. The spongy tissue is formed by elementary particles of living, which have an irregular shape, a large amount of intercellular substance and themselves very loose laid.
It takes part, but not as actively as the palisade parenchyma, gas exchange takes place in assimilation, as well as through its air spaces. Also in the leaflet are veins, which serve as vessels, participating in the metabolism. It is through them that water with minerals enters the cells of the lateral organ of the shoot, and from the leaf itself removes
organic compounds formed
during photosynthesis. Large veins are also surrounded by fibrous bundles formed by
mechanical tissue and give strength to the leaf.
Thus, the structure of the sheet is very complex and is due to the functions that this body performs - assimilation, gas exchange, guttation and evaporation. Also, in addition to the main ones, the leaf can also perform additional functions - protection (thorns), stock of substances (bulb scales) and vegetative propagation.