256 MORPHOLOGY 
as an advanced character, and is one of the prominent features of the 
families considered as highest. 
Irregularity. There is also often a tendency for one or more of the 
sets to become irregular (zygomorphic), a tendency noteworthy chiefly 
among petals. This means that the corolla is not composed of similar 
petals, as is true of regular (actinomorphic) flowers, but that the petals 
differ decidedly in form, as, for example, in a sweet pea. This tendency 
to irregularity is not a general one, but is characteristic of certain 
groups. 
These various tendencies are found in the different groups in all stages 
of development, so that the relative rank of a group is determined by the 
combination of its stages. For example, a naked, spiral, hypogynous 
flower, in which there is no zonal development, would have the most 
primitive combination; while a cyclic, sympetalous, syncarpous, and 
epigynous flower would have the most advanced combination. 
Organogeny. The organogeny of a flower has to do with the develop- 
ment of the floral members, the most noteworthy fact being the order of 
succession of the different sets. In a spiral flower the order of succes- 
sion is necessarily acropetal; that is, the sets arise successively towards 
the apex of the receptacle. This succession, therefore, is sepals, petals, 
stamens, carpels. If this succession is maintained in a cyclic flower, 
the acropetal succession, of course, appears centripetal. But with the 
shortening and broadening of the floral axis (receptacle), the primitive 
succession is often broken up. For example, in Compositae (the high- 
est family) the succession is petals, stamens, carpels, sepals, which is a 
striking shift in the position of the sepals; while in Capsella (shepherd's 
purse) the succession is sepals, stamens, carpels, petals. 
Relation of sporangia. The flowers of angiosperms are prevailingly 
bisporangiate ; that is, stamens and carpels occur in the same flower (fig. 
562). In the case of monosporangiate flowers two conditions are pos- 
sible : the staminate and carpellate flowers may occur upon the same 
plant (monoecious) or upon different plants (dioecious). 
STAMEN 
General character. The stamen is the organ bearing microsporangia, 
and therefore is the equivalent of the microsporophyll of gymnosperms. 
The sporangia are usually four in number, but they vary from one to 
many. Usually the stamen is differentiated into two distinct regions : 
the anther, which is the region bearing the sporangia; and the filament. 
