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2. Leaf Anatomy

Mesophyll and veins

The majority of photosynthesis takes place in the mesophyll between the upper and lower epidermis layers. Usually, the two layers of mesophyll can be distinguished from each other. The upper region is made of cells that look like short posts in two rows. These cells are parenchyma cells and make up the palisade mesophyll tissue. It is this tissue that contains more than 80% of the chloroplasts in the leaf. The lower layer of mesophyll, the spongy mesophyll tissue, is composed of loosely arranged parenchyma cells with abundant air space. The lower layer also contains many chloroplasts and its loose structure allows for movement of air in from the stomata. For future reference, parenchyma tissue containing numerous chloroplasts is called chlorenchyma tissue. It is also found in the outer parts of the cortex in the stems of herbaceous plants. However, in the leaf, the surfaces of the mesophyll that come into contact with the air are moist. The stomata will close if the internal moisture drops below a certain level in order to reduce drying inside the leaf.

The skeleton of a leaf is the veins (or vascular bundles). They are of various sizes and as described in the leaf arrangement section, are scattered throughout the leaf and are organized distinctly in different types of leaves. The veins are surrounded by a jacket of fibers called the bundle sheath. The sugars produced in the mesophyll are transported via the veins throughout the plant—specifically in solution in the phloem. In dicots, the veins run in all directions. In monocots, the veins are parallel and are not scattered. In addition, monocots do not have mesophyll differentiated into two layers. Instead, some will have large thin-walled buliform cells surrounding the main vein. The thin-walled cells are sensitive to water conditions and will collapse in dry conditions which causes the leaf blade to fold or roll which reduces transpiration (water loss).

Categories
2. Leaf Anatomy

Stomata

Stomata distinguish the lower epidermis from the upper epidermis. The upper epidermis is generally uninterrupted, but the lower epidermis is perforated by numerous tiny pores called stomata. The stomata (stoma singular) are very numerous and facilitate gas exchange between the interior of the leaf and the environment. Each stoma is regulated by a pair of sausage-shaped guard cells.

They, as mentioned earlier, are the only cells in the epidermis with chloroplasts for photosynthesis. The photosynthetic products in the guard cells provide the energy for the functioning of the cells. The walls of the guard cells are thickened, except for the side adjacent to the pore. The cells will expand or contract with changes in the amount of water in the cells, hence the need for energy as the water is moved into and out of the guard cells. When the guard cells are full of water the stoma pore is open and when the water is evacuated the pore is closed.

Categories
2. Leaf Anatomy

Epidermis

In cross-section, there are three major regions to see in the inside of a leaf: epidermismesophyll, and veins—or vascular bundles. The epidermal layer is one cell thick and covers the entire surface of the leaf. On the lower surface of the leaf blade, the epidermis is interrupted by stomata. Which will be discussed shortly. From the top, the epidermal cells look like jigsaw puzzle pieces fit tightly together. The guard cells in the lower epidermal layer contain chloroplasts, but otherwise, the epidermal cells do not have any chloroplasts and function as the primary protection for the cells beneath. Most leaves have a thin covering of waxy cuticle.

Categories
2. Leaf Anatomy

Leaf  Anatomy

plant leaf layers
Schematic diagram of the anatomy of a leaf. )