Leaf Development

Leaf Development

Leaves are initiated from groups of founder cells close to the stem apex. These undergo periclinal divisions either in the outer-most cell layers or in the layers immediately below them, to form small conical projections (leaf primordia). In monocots the leaf primordium rapidly develops into a bifacial hood-like structure, and the base of the primordium partially or wholly encircles the stem, forming a leaf sheath. Within a simple leaf, most subsequent meristematic activity occurs in a highly plastic transition zone between the precursor tip and sheath. One or more adaxial cross (transverse) meristems in the transi-tion zone give rise to many other structures such as ligules and stipules.

The adaxial marginal cells divide rapidly to form a flattened leaf blade. This marginal growth is suppressed in the region that later becomes the petiole, and in many monocots it often occurs at the same time as apical growth. Marginal growth is subsequently replaced by cell divisions across the whole leaf blade; by this stage the approximate number of cell layers has been established, and the whole lamina functions as a plate meristem. Cell divisionsare mainly anticlinal, resulting in regular layers of cells thatare disrupted only by the differentiation and maturation of the vascular bundles.

Rates of growth and cell division sometimes vary in different parts of the leaf. Individual leaflets of a compound leaf may be produced either acropetally or basipetally. For example, in the tomato (Solanum lycopersicum), the uppermost (distal) leaflets are initiated first, followed by middle and lower leaflets(i.e. a basipetal sequence). Smaller intermediate leaflets are formed later, in a more chaotic sequence. In some monocots, meristematic activity governing leaf elongation is restricted to a region at the base of the leaf, the basal rib meristem; this results in axial files of cells that increase in maturity towards the distal end of the leaf. The unifacial leaves of some monocots (e.g. Acorus), which possess a bifacial sheathing leaf base and a unifacial upper blade, result from suppressed marginal growth and the presence of an adaxial (ventral) meristem in the transition zone.