DIV. n 



SPERMATOPHYTA 



541 



The development of the pollen sac (Fig. 509) commences with 

 divisions parallel to the surface taking place in cells of the hypodermal 

 layer; this separates the cells of the primary archesporium from an 

 outer layer of cells. The latter divides to form three layers of cells. 

 The outermost layer of the wall in Gymnosperms and the hypodermal 

 layer in the Angiosperms gives rise to the fibrous layer and the inner- 

 most layer to the tapetum. The archesporium after undergoing a 

 number of divisions forms the pollen-mother-cells, each of which divides 

 as in Bryophytes and Pteridophytes into four daughter cells. These 



FIG. 509. Hemeroccdlis fulva. A, Transverse section of an almost ripe anther, showing the loculi 

 ruptured in cutting ; p, partition wall between the loculi ; a, groove in connective ; /, vascular 

 bundle ( x 14). B, Transverse section of young anther ( x 28). C, Part of transverse section of a 

 pollen sac ; pm, pollen-mother-cells ; t, topetal layer, later undergoing dissolution ; c, inter- 

 mediate parietal layer, becoming ultimately compressed and disorganised ; /, parietal layer of 

 eventually fibrous cells; e, epidermis (x 240). D and E, Pollen-moth er-cells of Alchemilla 

 speciosa in process of division (x 1125). F, Mature tetrad of Bry&nia dioica ( x 800). (After 

 STRASBURGER). 



are the pollen grains, and are spherical or ellipsoidal in shape and 

 provided with a cell wall ; an external cutinised layer (the EXINE), 

 and an inner cellulose layer, rich in pectic substances (the INTINE), 

 can be distinguished in the wall. 



While the male sexual cells of all archegoniate plants are depend- 

 ent on water for their conveyance to the female organs, the transport 

 of the pollen grains to the egg-cells is brought about in Spermatophytes 

 by means of the wind or by animals. However far the reduction of 

 the male prothallium has proceeded and even in the case of the 

 heterosporous Pteridophyta only a single sterile cell was present two 

 constituent parts are always distinguishable in the germinating pollen 

 grain ; these are a VEGETATIVE CELL which grows out as the POLLEN- 



