Mesophyll

Mesophyll

Chlorophyll is contained in chloroplasts in the mesophyll, which is the primary photosynthetic tissue of the leaf. In many plant species the mesophyll is divided into distinct regions (termed palisade and spongy tissues), though in others it is rela-tively undifferentiated and homogeneous throughout the leaf. Palisade mesophyll is typically adaxial. Palisade cells are anticlinally elongated and possess relatively few intercellular air spaces. By contrast, spongy mesophyll is typically located on the abaxial side of the leaf, and consists of variously-shaped cells with many air spaces between them. Both palisade and spongy tissues can be up to several cell layers thick. Occasionally there is intergrada-tion between the two tissues.

Many tropical grasses, and also some other taxa that are only distantly related (both monocots and dicots), possess a ring of mesophyll cells radiating from the vascular bundles (Kranz anatomy;). This structure is commonly associated with the C₄ pathway of photosynthesis (Fig. 4.11). In thick leaves, particularly those of some monocots, the central cells are often large, undifferentiated and non-photosynthetic. In the thick ‘‘keel’’ or ‘‘midrib’’ of Crocus leaves, a region of large parenchy-matous cells with their walls often broken down to form a cavity causes the characteristic white stripe along the centre of the leaf (Fig. 4.8).

In some xeromorphic plants (e.g. Ilex and Ficus: Fig. 4.4) subdermal layers immediately within the adaxial (or, more rarely, the abaxial) epidermis are modified into a hypodermis. This consists of one or more layers of non-photosynthetic cells that are usually slightly larger and thicker-walled than the adjacent

mesophyll cells and in transverse section frequently resemble epidermal pavement cells. Hypodermal cells become lignified in some older leaves.