CRYPTOGAMS AND PHANEROGAMS. 



245 



of the pollen-tube (the sperm-nucleus, male pronuclens) with that 

 of the oosphere. 



In the ANGIOSPERMS the reductions proceed still further. The 

 barren cell, which represents the prothal- 

 lium, was in the last group separated from 

 the antheridium by a true cell-wall, but in 

 the Angiosperms a membrane at most, but 

 no firm cell-wall, is formed. The pollen- 

 grain contains two cells, a vegetative and a 

 free generative cell. Both these pass into 

 the pollen-tube, but the vegetative cell dis- 

 appears about the time the pollen-tube 

 reaches the ovule ; while the generative 

 cell divides into two: one, the sperm-nucleus 

 coalescing with the nucleus of the oosphere, 

 the other being absorbed (Lilium, after 

 Guinard). 



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 for the emergance of the pollen-tube. 



2. The Macrospores. The prothallium in Salviuia 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 J/iam7ia only one, in tialvinia 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 

 Isoetcs and Seh.ii/iueUa 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- 



PIG. 250. I Pollen-grains 

 of Cupressus ; at the top is 

 seen one proth;illium-cell. 

 II Germinating ; a pollen- 

 tube; a the extine ; li the 

 inline. 



