The leaf is a collection of tissues which include:
- The epidermis which covers the upper and lower surfaces.
- The mesophyll inside the leaf which is rich in chloroplasts.
- The veins contains the vascular tissue (where xylem and phloem are present).
Epidermal cells form the outer layer covering a leaf, separating internal tissues from the external environment.
Epidermis tissue has several functions:
- protection against water loss via stomata and a waxy cuticle
- regulation of gaseous exchange
- secretion of metabolic compounds
The mesophyll is located between the upper and lower layers of the leaf epidermis, and is mostly made up of parenchyma (ground tissue) or chlorenchyma tissue. The mesophyll is the primary location for photosynthesis and is divided into two layers, the upper palisade layer and the spongy mesophyll layer.
The upper palisade layer lies beneath the upper epidermis and consists of vertically elongated cells that are tightly packed together to maximise the number of cells exposed to sunlight. In addition, these cells contain many chloroplasts, thus maximising their photosynthetic ability. The palisade layer thickness depends on the extent of exposure to the sun. Leaves that are exposed to the sun have a thicker palisade layer. Those that are typically found in the shade have a thinner palisade layer. Beneath the upper palisade layer is the spongy mesophyll. The cells in the spongy mesophyll are slightly rounder and less densely packed and have air spaces to allow for gaseous exchange.
Figures below show the leaf and tissue structure of a dicot plant.
Vascular tissue is made up of the xylem and phloem vessels you learnt about earlier in this chapter. Xylem transports water and minerals to the leaf. Phloem transports dissolved sucrose made in the leaf out of its site of synthesis to the rest of the leaf. Most leaves have a bundle sheath around the xylem and phloem, consisting of sclerenchyma or collenchyma, for extra support.