Dicotyledonous leaf structure

The dicot leaf consists of one or more layers of densely packed parenchyma cells rich in chloroplasts, and a variety of supporting tissues that allow this thin layer of cells to perform photosynthesis efficiently. The dense layer of parenchyma is called palisade mesophyll. Its cells are stacked tightly together, with their long axes perpendicular to the top surface of the leaf. Each is directly exposed to light at its top end. The bottom end is in contact with a loosely packed layer of spongy mesophyll, with numerous air spaces between the cells. This airy tissue allows rapid diffusion of atmospheric gases.

The micrograph above shows a typical dicot leaf. The palisade and spongy mesophyll layers are sandwiched between two transparent layers of epidermis. The epidermis of the leaf produces a waxy cuticle that waterproofs the leaf. This prevents the otherwise catastrophic loss of water that would occur if such a large area of moist cells were exposed to the air, but it creates a new problem. Carbon dioxide and oxygen cannot cross the cuticle. Small pores called stomata (or stomates), usually on the moister, cooler lower epidermis, allow gases to cross the epidermis. These are protected by paired cells called guard cells, that can change shape to open or close the pores, depending on the availability of water and the rate of evaporation. The micrograph below shows a close up of the leaf, in which two stomata are visible (one open, one closed).

Veins rise from the vascular bundles of the stem and enter the leaf through the leaf stalk or petiole. In dicots the veins then branch out in a net-like pattern to reach all parts of the leaf blade. Each vein contains xylem, which brings in water and simple minerals, phloem, which carries sugars from the photosynthetic cells to all other live cells in the plant, and fibres which help to support the otherwise flimsy leaf blade.

The micrograph below shows more detail of a major leaf vein. Note that the xylem is found on the top and the phloem on the bottom.