SPERM A TOPH YTA 38 1 



beneath the epidermis, then undergo further perichnal divisions, usually- 

 forming about three to five layers of parietal tissue (Fig. 325C, D) . The 

 outermost parietal layer, lying next to the epidermis, is the endothecium. 

 As a rule, by the development of fibrous bands of thickening, the endo- 

 thecium becomes hj^groscopic and assists in the dehiscence of the anther 

 (Fig. 324B). The innermost layer of parietal tissue forms part of the 

 tapetum, the rest of which is derived from the cells immediately in con- 

 tact with the sporogenous cells on their inner side. Sometimes the 

 tapetum becomes two- or three-layered. An interesting feature is the 

 division of the tapetal nuclei to form two or more free nuclei in each cell 

 (Fig. 32oZ)). The middle layers and tapetum generally disappear before 

 the maturing of the spores, the ripe sporangium wall usually consisting 

 only of the epidermis and endothecium. A tapetal Plasmodium, sur- 

 rounding the microspores, is seen in several groups, such as the Compositae 

 and Helobiales. 



The cells forming the primary sporogenous layer generally undergo two 

 or three divisions to form the microspore mother cells, which then greatly 

 enlarge and assume a spherical form. The next two divisions, during 

 which the number of chromosomes is reduced one-half, result in the for- 

 mation of tetrads. At this time the tapetum disorganizes. The tetrads 

 are mostly tetrahedral in dicotyledons and isobilateral in monocotyledons. 

 A linear arrangement of microspores is rare, but occurs in the milkweeds 

 (Asclepias) and a few other forms. 



Upon separation from the tetrads, the microspores have developed a 

 two-layered cell wall consisting of an outer exine and an inner inline (Fig. 

 334). Although commonly the exine is thickened, the cell wall usually 

 has one or more thin places where the exine is not formed and through 

 which the pollen tube may later emerge. Its outer surface usually bears 

 warts or spines, or is variously sculptured. Ordinarily the microspores 

 become free from one another at maturity, but in some angiosperms {e.g., 

 Typha and Rhododendron) the members of the tetrad do not separate, 

 while in a few others, notably in the milkweeds (Asclepias) and certain 

 orchids, all the spores in a sporangium cling together and escape as a mass, 

 which is called a pollinium. 



As a rule, the anther dehisces by means of two longitudinal slits (Figs. 

 318B and 324J5), but sometimes by terminal slits or pores, by hinged 

 valves, or irregularly. 



The Carpel. The carpel of angiosperms is really a megasporophyll but, 

 instead of bearing the ovules freely exposed, as in gymnosperms, it sur- 

 rounds them. Where two or more carpels are wholly or partly united, 

 forming a compound pistil, the flower is said to be syncarpous. Where 

 the carpels are free, each constituting a simple pistil, the flower is apocar- 

 pous. The enlarged, hollow, lower portion of the pistil, the ovary, encloses 



