22.6Formation of Leaves
There is a cell population in the circumference of the shoot apical meristem that grows and protrudes out of the meristem through active cell division. This cell population is called the leaf primordium. Auxins that are acropetally transported at the epidermal layer of the stem are essential to start the formation of the leaf primordium. The leaf that had developed previously intervenes in determining the location where the next leaf primordium is to be formed. This contributes to the regularity of the positional relationship between sequential leaves, creating a mathematically exquisite arrangement of leaves. Although, the intervening actions involved here are still not clear, one promising hypothesis proposes that the preceding leave influences the distribution of auxins, which is thought to play an important role.
Leaves are zygomorphic organs. Differentiation between the adaxial (upper) and abaxial (lower) surfaces holds the key to this morphogenesis. Leaf primordia that have lost contact with the shoot apical meristem only have an abaxial structure. Thus, the leaves are virgulated and radially symmetrical. This indicates that some sort of signal is sent out from the shoot apical meristem, which induces characteristics that are unique to the adaxial side of the leaf primordium (Figure 22-7A). Once the abaxial and adaxial sides are distinguished in the leaf primordium, the borderline between the two sides becomes the leaf margin, which will grow to become the blade. When heterotopic abaxialization or adaxialization occurs (for example, when a zone with abaxial characters occurs as a mottle on the adaxial side of the leaf) due to a sudden mutation, growth progresses abnormally at the area where the zones with adaxial and abaxial characters contact each other, forming a structure like that of a leaf blade (Figure 22-7B).