In this article we will discuss about the structure of a leaf with the help of a diagram.
A leaf is a compromise between two conflicting evolutionary pressures. The first is to expose a maximum photosynthetic surface to sunlight; the second is to conserve water while, at the same time, providing for the exchange of gases necessary for photosynthesis.
The photosynthetic cells of leaves are of a general type known as parenchyma. They are many-sided cells with thin, flexible cell walls. In many leaves, there are two types of parenchyma cells-palisade parenchyma, consisting of long columnar cells in which most of the photosynthesis takes place, and spongy parenchyma, which consists of more rounded cells with larger air spaces surrounding them.
These spaces are filled with gases, including water vapor. Palisade and spongy parenchyma make up the mesophyll, or “middle leaf.” The mesophyll is completely enclosed in an almost airtight wrapping made up of epidermal cells. These cells secrete a fatty substance called cutin, which forms a coating, the cuticle, over the outer surface of the epidermis.
The epidermal cells and the cuticle are transparent, permitting light to penetrate to the photosynthetic cells of the mesophyll. Materials move into and out of the leaf by two quite different routes- veins and stomata. Veins supply water and minerals to the leaf cells.
The veins pass through the petioles (leaf stalks) and are continuous with the rest of the vascular system of the plant. (Note that they are called veins only when they are in the leaf; in the stem and root these same structures are called vascular bundles. It is correct to refer to veins as vascular bundles but not vice versa.)
The tissue in vascular bundles that is concerned primarily with water transport is known as the xylem. Other groups of cells, the phloem (pronounced “flow-em”), transport sugars and other products from the, photosynthetic (autotrophic) cells to the other (heterotrophic) cells of the plant.
Veins form distinctive patterns in leaf blades. There is a conspicuous difference between the veins of monocots and dicots: In monocots, the major veins are usually parallel; in dicots, the venation (vein pattern) is netted, either palmate (fan-like) or pinnate (feather-like), as shown in Figure 13.1.
Gases-oxygen and carbon dioxide moves into and out of the leaf by means of stomata. (A stoma, as we define it, includes both the opening, or pore, and the guard cells surrounding it.) This exchange of gases is necessary for photosynthesis.
However, as gases are exchanged, water escapes from the leaf; 90 percent of the water that leaves the plant body is lost through the stomata. The stoma opens and closes in response to a variety of stimuli, both internal and external.
Stomata may be very numerous; for example, in tobacco leaves, there are 12,000 stomata per square centimeter of leaf surface. They are usually most abundant on the undersurface.