The main function of the leaf is to carry out photosynthesis.
Photosynthesis is a process by which green plants use sunlight to produce organic matter from inorganic substances. Photosynthesis takes place in chloroplasts. Familiarity with the internal structure of the leaf blade will help to better understand the meaning of green leaves in plant life.
Above and below the leaf is covered with a thin transparent skin (epidermis, or epidermis), its cells protect the leaf from damage and drying. The skin is one of the types of plant integumentary tissue.
The skin cells are colorless so that sunlight can freely penetrate into the leaf. The top of the skin is covered with a waxy layer (wax coating) that prevents water loss – the cuticle (plants with thick cuticles evaporate less water than plants with thin cuticles).
Among the cells of the skin there are stomata, through which air enters the leaf and water evaporates.
The structure of the pulp of the leaf.
Under the skin is the pulp of the leaf, which consists of the cells of the main tissue. In the cells of the main tissue, there are chloroplasts in which photosynthesis takes place. Two or three layers, directly adjacent to the upper skin, are formed by elongated cells tightly adjacent to each other. They resemble columns of almost the same size, therefore the upper part of the main leaf tissue is called columnar. There are especially many chloroplasts in the cytoplasm of these cells.
More rounded or irregularly shaped cells lie under the columnar tissue. They are loosely adjacent to each other, forming intercellular spaces. The intercellular spaces are filled with air. There are fewer chloroplasts in these cells than in the cells of the columnar tissue. These cells form spongy tissue.
Leaf vein structure
If viewed under a microscope, a transverse section of a leaf blade, one can see in it the conducting bundles of the leaf – veins consisting of vessels, sieve tubes and fibers.
Strongly elongated cells with thick walls – fibers – give strength to the sheet. Water and minerals dissolved in it move through the vessels. Solutions of organic substances move along the sieve tubes.