CRYPTOGAMS AND PHANEROGAMS. 



245 



cell without cilia. After the rupture of the membrane at the 

 tip of the pollen-tube they pass over into the archegonium, reach 

 the oosphere and complete the fertilization, the sperm-nucleus 

 fusing with the nucleus of the oosphere. 



In the ANGIOSPERMS the reduction pro- 

 ceeds still further. The pollen-grain contain s 

 two cells, a vegetative and a small an- 

 theridial cell, but these are not, as in the 

 last group, separated by a true cell- wall ; a 

 membrane at most being formed between 

 them. Both these cells pass into the pollen- 

 tube, but the vegetative cell disappears 

 about the time the pollen-tube reaches the 

 ovule : while the antheridial cell divides 

 into two : one, the sperm-nucleus, coales- 

 cing with the nucleus of the oosphere, the 

 other uniting with the definitive nucleus. 



The Gymnosperms prove in yet another 

 point that they are more closely related to 

 the Cryptogams than are the Angiosperms. 

 When the pollen -grain begins to germinate 

 the external wall ruptures as in the Cryp- 

 togams (Fig. 250), but in the Angiosperms special germ-pores are 

 formed in the cell-wall f6r the emeigence of the pollen-tube. 



2. The Macrospores. The prothallium in Salvinia and Marsilia 

 is still rather large, green, and capable of the independent assimi- 

 lation of carbon. It projects more or less from the macrospore and 

 bears (in Marsilia only one, in Salvinia several) archegonia, which 

 however are embedded to a greater degree in the prothallium, and 

 are more reduced than the archegonia of the true Ferns and Horse- 

 tails (Figs. 215, 216). The prothallium is still more reduced in 

 Isovtes and Selaginella ; partly because it is smaller and is in a higher 

 degree enclosed in the spore, it also contains less chlorophyll, or 

 is entirely without chlorophyll, and in consequence incapable of 

 independent existence, whilst the number of archegonia is less ; and 

 partly because the archegonia are themselves reduced, the cells of 

 the neck are fewer and embedded to the level of the surface 

 of the prothallium without any, or with only a very slight 

 projection (Figs. 235, 236). Finally, the prothallium with its arche- 

 gonia begins to develope in Selaginella while the macrospore is still 

 within its sporangium, and before it is set free from the mother- 



FIG. 250. I Pollen-grains 

 of Cupressus ; at the top is 

 seen one prothallinm-cell. 

 II Germinating; c pollen- 

 tube ; a the exine ; b the 

 inline. 



