This definition is free from temporal constraints, it will allow metamers to exhibit great modification, and it will enable comparisons to be made among sequences.
For every given species, then, one can define the RGU in terms of the kinds of metamers produced by the meristem and by the sequence in which they are produced.
Sylleptic buds are those buds that continue development while the RGU on which they are formed is still completing development itself: that is, before the complete sequence of metamers has been produced.
For ease of presentation the observed heterochronic differences in inflorescence structure may be divided into three major types: observed heterochronic differences in the relationship between the unit inflorescence(s) and the subtending leaf (leaf hysteranthy); observed differences in the time of formation and/or duration of whole axes; and dimorphic axes or shoots, in which is one RGU may have metamers modified differently from those of other RGUs.
Spatial hysteranthy implies that heterochronic development of the leaves subtending peduncles takes place only along some (either the distal or proximal) of the nodes of the subsequence or fertile metamers and that some are fully suppressed.
In the first, the fertile metamers are acrotonic, or produced at the terminal, younger, nodes of the RGU.
Heterochrony was assumed to be the cause of the four above-mentioned differences, with no empirical study to determine whether structural modification of any metamers might account for the differences.
Finally, some observed differences in inflorescence structure involve shoot dimorphism: axes on one branch, RGU, or subunit have metamers modified differently from metamers on other branches or RGUs.