Most photosynthesis takes place in the mesophyll between the two epidermal layers, with two regions often being distinguishable. The uppermost mesophyll consists of compactly stacked, barrel-shaped or post-shaped parenchyma cells that are commonly in two rows. This region is called the palisade mesophyll and may contain more than 80% of the leaf's chloroplasts. The lower region, consisting of loosely arranged parenchyma cells with abundant air spaces between them, is called the spongy mesophyll. Its cells also have numerous chloroplasts.
Parenchyma tissue with chloroplasts is called chlorenchyma. Chlorenchyma tissue is also found in the outer parts of the cortex in the stems of herbaceous plants as well as in leaves. Inside the leaf, the surfaces of mesophyll cells in contact with the air are moist. If the moisture level decreases below a certain point, the stomata close, thereby significantly reducing further drying.
Veins (vascular bundles) of various sizes are scattered throughout the mesophyll (Fig. 7.9). They consist of xylem and phloem tissues surrounded by a jacket of thicker-walled parenchyma cells called the bundle sheath. The veins give the leaf its "skeleton." The phloem transports throughout the plant carbohydrates produced in the mesophyll cells. Water, sometimes located more than 100 meters (330 feet) away in the ground below, is brought up to the leaf by the xylem, which, like the phloem, is part of a vast network of "plumbing" throughout the plant. Since veins run in all directions in the network, particularly in dicots, it is common when examining a cross section of a leaf under the microscope, to see veins cut transversely, lengthwise, and at a tangent, all in the same section.
thickened inner wall of guard cell
— stoma nucleus chloroplast guard cell thickened inner wall of guard cell
— stoma nucleus chloroplast guard cell
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