hmm...
"The meristem continually produces new leaves, which are visible as they initiate as slight bumps, or
primordia, at the periphery of the meristem. Leaves are initiated at a certain frequency and in a certain pattern [fig. 9.5]. The timing between successive leaf initiations is called the
plastochron and can be described in stages of leaf development from initiation to full differentiation. The plastochron stages are numbered P1, P2, P3, . . . , Pn, where n is the number of leaves differentiating and varies widely between different plant species. The leaf continues going through plastochron stages until it is fully differentiated. The pattern in which new leaves are initiated is
phyllotaxy. There are two possible phyllotactic patterns:
spiral phyllotaxy, where leaf primordia bud at certain angles from the previous primordia, and
whorled phyllotaxy, where leaf primordia form in alternating patterns of one (distichous), two (decussate), or three (tricussate) leaves per whorl.
There are many models to explain how phyllotactic patterning occurs, most falling under two concepts: morphogenetic fields or biophysical constraints. The concept of morphogenetic fields and related models suggest that an initiated primordia makes a diffusible substance that inhibits the initiation of another primordia, so that the next primordia will form only after escaping this biochemical constraint. Alternatively, the concept of biophysical constraints suggests that the positions of existing primordial effect initiation of new primordia by limiting the available physical space. The
available space model can include either biochemical or biophysical constraints and is supported by studies of mutants producing more organs than normal because they have larger meristems, and thus more room to escape biochemical or biophysical constraints [fig. 9.6]. However, these mutants do not change the phyllotactic pattern of leaves, only the number of leaves produced in that pattern. Surgical manipulations can be performed in corn that will change phyllotactic patterning from whorled to spiral [fig. 9.7]. This mimics the natural change in phyllotactic patterning that occurs in corn during the progression from leaf formation (whorled pattern) to flower formation (spiral pattern). "
Principles of Developmental Biology
