CONTROL OF ORGANOGENESIS AT THE SHOOT APEX

The processes involved in the formation of primordia on the shoot apex are those controlling (1) growth rate, (2) division plane, (3) surfac e microstructure, and (4) extensibility of the surface. Changes in gro wth rate and division planes may accompany primordium formation but ar e considered as probably not in themselves being causal. Changes in su rface microstructure may be necessary to delimit the position and area occupied by an incipient primordium. However, attention is directed t o changes in surface extensibility as perhaps being the overriding fac tor in primordium formation. Nevertheless, the position and form of th e primordia will also depend on growth rate, division plane, and surfa ce microstructure being permissive. The relative importance of these f our sets of processes may differ from species to species and from one stage of development to another. Chemical and metabolic changes within the apex may first be necessary to determine whether the surface can extend sufficiently for any primordia to form at all, but their positi ons and time of initiation may depend more on the other factors. The s urface microstructure may become more important when patterning is det ailed and precise as it is in the developing flower, whereas a less pr ecise mechanism dependent on localized induction of synthesis of a mor phogen (auxin !) may provide sufficient information to determine the g eneral position and timing of primordium initiation in vegetative apic es. In determining the pattern of primordia on the apex, primordial ar ea at initiation is important and reasons for believing that auxin may be involved in determining this are summarized. The different develop mental pathways of primordia seem to diverge from the moment of initia tion. Developmental fate of primordia is determined by the homeotic ge nes which may in fact be heterochronic genes. How these regulatory gen es control the processes involved in differentiation of different type s of primordia is so far unknown.