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 th 

 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. 



