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.