STRUCTURE OF MICROSPORE 377 



Above the sepals comes a whorl of leaves (per 2 ), usually 

 of large size and bright colour, forming in fact the most 

 obvious part of the flower. These are the petals and together 

 constitute the corolla. The calyx and corolla together are 

 conveniently called the perianth, because they inclose the 

 sporophylls or essential part of the flower. The presence of 

 a well-marked perianth is characteristic of the majority of 

 Angiosperms and distinguishes them from Gymnosperms in 

 which this part of the flower is quite rudimentary (see Fig. 

 88, A and B, per). 



The third whorl is called collectively the andrcecium and 

 consists of a variable number of stamens or male sporo- 

 phylls (sp.ph. $ ). Each stamen is a long narrow leaf bearing at 

 its distal end four microsporangia (pollen sacs, mi. spg) united 

 into a lobed knob-like body, the anther. . The microspores 

 (c 1 ) are at first simple cells with double cell-walls, but 

 subsequently the protoplasm becomes divided into two cells, 

 as in Gymnosperms, a smaller (a) and a larger ($). The two 

 are not, however, separated by a firm septum of cellulose 

 and the smaller cell frequently comes to lie freely in the 

 protoplasm of the larger. Moreover it appears that the 

 nucleus of the smaller is the active agent in fertilization and 

 that the larger must therefore be considered as representing 

 the vestigial prothallus. 



The fourth or distal whorl of the flower is called collec- 

 tively the gyncecium or pistil, and consists of one or more 

 carpels or female sporophylls (sp. ph. $ ), which are modified 

 in a characteristic manner. In some cases each carpel (B 1 , 

 B 2 ) becomes folded longitudinally along its midrib (mr), and 

 its two edges, thus brought into contact, unite so as to 

 inclose a cavity. Concrescence only affects the proximal 

 part of the carpel, which thus becomes a hollow capsule, the 

 venter (so-called ovary, A, v) : its distal portion usually takes 



