SECT. II 



PHANEROGAMIA 



429 



(c/. pp. 412, 416). The flowers of the Gymnosperms in fact show but a 

 small advance from the flower-cones of the Pteridophytes, while those of 

 the Angiosperms differ from them only in the more pronounced metamor- 

 phosis of their various parts. A rose, or the complicated flower of an 

 Orchid, represents the more highly developed forms of an ascending 

 but continually diverging series, which originated in the Pteridophytes. 

 The first indication of a tendency to form a flower is manifested by 

 some of the Ferns, e.g. Blechnum, in which the fertile leaves, separated 

 from the sterile, are united in a rosette crowning the apex of the axis. 

 The microsporangia of Phanerogams are termed pollen-SACS ; the 

 microspores, pollen-grains, or collectively pollen. The development 

 of the pollen-sacs and pollen-grains (Fig. 358) is effected in the same 

 way as the homologous reproductive organs of the Pteridophytes. A 



./• 





fair,' 



Fig. 35S. — Hemerocctllis fulvc . 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, tapetal 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 mother-cells after division 

 (x 240). 



cell layer, directly under the epidermis of the sporophylls, becomes 

 divided by tangential walls into two layers, the outer of which 

 constitutes the wall of the sporangium, the inner the spore mother- 

 cells. The latter, by repeated division, give rise to the pollen 

 mother-cells, which further divide each into four pollen-grains. 

 Although the pollen-grains sometimes remain united in tetrads 

 (Fig. 359, A), they are generally isolated, and have the appearance of 

 round or elongated bodies, which are at first unicellular (Fig. 359, B, 

 360), but eventually, in consequence of the formation of a reduced 

 male prothallium, become multicellular. 



