SPERMATOPHYTA 375 



in spiral arrangement (Fig. 319). Such a condition is similar to that of a 

 strobilus. In most flowers, however, the receptacle does not elongate but 

 generally broadens at the apex, the floral parts arising from it in a series 

 of whorls. The members of one whorl usually alternate with those of the 

 next whorl (Fig. 318D). Commonly there are two whorls of perianth 

 parts, two whorls of stamens, and one whorl of carpels. Such flowers are 

 said to be pentaajdic. Where one whorl of stamens is wanting, this 

 being nearly always the inner one, the flower is tetracyclic. With the 

 establishment of a cyclic arrangement of floral parts, the members of each 

 set are reduced to a definite number that is often the same in all whorls. 

 In monocotyledons the number of parts in each whorl is generally three, 

 while in dicotyledons it is usually five but often four. In many flowers 

 the number of carpels is less than the number of parts in any of the other 

 whorls. 



Zonal Development. A striking feature of floral evolution has been the 

 tendency for the members of the same whorl to develop as a single organ. 

 Thus, in some flowers, the carpels are separate, each forming a simple 

 pistil, while in most flowers the carpels are organized to form a compound 

 pistil (Figs. 3185, C, and 326). Similarly, in many flowers, the petals are 

 wholly or partly united to form a corolla tube and the sepals are united to 

 form a calyx tube. Obviously a sijncarpous flower (one with united car- 

 pels) is more advanced than an apocarpous one (one with separate car- 

 pels) , and a sympetalous flower (one with united petals) is more advanced 

 than a choripetalous one (one with separate petals) . In some flowers the 

 stamens are united to form a tube, but this condition is uncommon. 



It should be understood that, in all flowers where members of the same 

 set are united, the parts do not arise separately and later fuse, but origi- 

 nate together from a common meristem and develop as a single organ. 

 There is a zonal development from the receptacle that involves all mem- 

 bers of the same set, so that they are united from the beginning. Some- 

 times the primordia of the individual members arise separately but are 

 soon carried upward by zonal development from below. This results in a 

 compound pistil with separate styles or in a corolla tube with free tips. 



Hypogyny, Perigyny, and Epigyny. In most sympetalous flowers the 

 stamens are free above but are attached below to the corolla tube. Here 

 zonation involves the members of two different sets. A still more 

 advanced condition occurs where the receptacle enters into a zonal devel- 

 opment with other floral sets. In hypogynous flowers all the sets arise 

 independently from a more or less convex receptacle, the ovary being 

 entirely free and situated above the place of attachment of the sepals, 

 petals, and stamens (Fig. 320). In perigynous flowers the ovary is also 

 free, but the receptacle is more or less concave, forming a disk-like or cup- 

 like structure from the rim of which the sepals, petals, and stamens arise. 



