The sheet is a very important organ of the plant. It's part of the escape, the main functions of which are transpiration and photosynthesis. Features of the structure of a sheet lies in its high morphological plasticity, great adaptive features and variety of forms. The base can be expanded in the form of stipules – leaf-like oblique formations on each side. In some cases they are so large that photosynthesis play a role. The stipules are adnate to petiole or free, they can be offset on the inner side, and then called axillary.
External structure of the sheet
Leaf blades are not the same size: they can be from a few millimeters to ten to fifteen meters, and even twenty meters from the palm trees. The structure of the leaf determines the lifespan of the vegetative organ, it is usually short - no more than a few months, although for some it is from one and a half to fifteen years. The shape and size are hereditary signs.
Parts of leaves
Leaf is a lateral vegetative body that grows from the stem, is at the base of the growth zone, and bilateral symmetry. It usually consists of a petiole (except sessile leaves), and leaf blade. A number of families, the structure of the sheet also requires the presence of stipules. The external organs of plants can be simple – with one plate, and complex – with multiple records.
A leaf pad (base) is the part that connects the leaf with the stem. The educational tissue that is located here gives rise to the stem and leaf blade.
Petiole - narrowed part, connecting the stem and leaf blade with its base. He orients the sheet with respect to light, acts as the place where the inserted educational tissue is located, due to which the vegetative organ grows. In addition, the scape weakens impacts on the leaf during rain, wind, hail.
The lamina plate is usually a flat, extended part that performs the functions of gas exchange, photosynthesis, transpiration, and in some species also the function of vegetative reproduction.
Speaking about the anatomical structure of the leaf, it is necessary to say about stipules. These are leaf-shaped paired formations at the base of the vegetative organ. When you deploy a sheet, they may fall off or be saved. Designed to protect the axillary lateral buds and intercalary educational tissue.
Complex and simple leaves
The structure of a leaf is considered simple if it has one leaf plate, and difficult - if there are several or many plates with joints. Due to the last plates of complex leaves fall not together, but one by one. But some plants may fall completely.
If the plate is elongated, with triangular segments and lobes, the leaf is called strugo-like (for example, a dandelion). If the lateral lobes to the base decrease, they are unequal, and the final lobe is round and large, a lyre outer organ of the plant is obtained (for example, in a radish).
A sheet structure with multiple plates is significantly different. Allocate are palmated, Troisfontaine, peristerona bodies. If the complex sheet contains three records, it's called trifoliate, or troisfontaines (e.g., maple). Are palmated leaf is counted when it petiolules are attached to the main petiole at the same point, and the plate diverge radially (e.g., lupine). If the side plates on the main stalk on both sides along the length of the sheet called Perestoronin.
Forms of whole plates
Different plants have different forms of leaf blades in the degree of dissection, shape, type of base and apex. They can have round, oval, triangular, elliptical and other outlines. The plate is elongated, and its free end can be blunt, pointed, sharp or pointed. The base is drawn and narrowed to the stem, it is heart-shaped or rounded.
Attaching to the stalk
Considering the structure of a leaf of a plant, it is necessary to say a few words about how it is attached to the shoot. The attachment is carried out using long or short stalks. There are also sessile leaves. In some plants, their bases grow together with a shoot (a decaying leaf), and it happens that a shoot escapes through the plate (pierced leaf).
Internal structure. Skin
The epidermis (upper skin) is a covering tissue located on the reverse side of the plant's organ, often covered with cuticle, hairs, and wax. The internal structure of the sheet is such that outside it has a skin that protects it from drying out, mechanical damage, penetration of pathogens to internal tissues and other adverse effects.
Peel cells are alive, they are different in shape and size: some are transparent, large, colorless, tight to each other; others are smaller, with chloroplasts giving them a green color, such cells can change shape and are arranged in pairs.
The skin cells can move away from each other, in which case a gap appears between them, which is called the stomatal. When the cells are saturated with water, the stoma opens, and when the fluid flows out, it closes.
The anatomical structure of the leaf is such that air flows through the stomatal gaps to the inner cells and gaseous substances go out through them. When the plants are not sufficiently supplied with water (this happens in hot and dry weather), the stomata close. So representatives of the flora protect themselves from draining, because with closed stomatal crevices water vapor does not come out and remain in the intercellular spaces. Thus, in the dry period, plants retain water.
The internal structure of the sheet is not complete without columnar tissue, which cells are at the top, turned to the light side, closely adjacent to each other have a cylindrical shape. All cells have a thin membrane, nucleus, chloroplasts, cytoplasm, vacuole.
Another main fabric is spongy. Its cells are round in shape, loosely arranged, between them there are large intercellular spaces filled with air.
The structure of the leaf of a plant, how many layers of spongy and columnar tissues are formed, depends on the lighting. In the leaves grown in the light of the leaves, the columnar tissue is much more developed than in those that grew under darkening conditions.