Seed-Plants 125 



megasporangium of Selaginella. Usually but one 

 spore matures, but this is very large, and within it 

 arises the gametophyte very much like that in the 

 megaspore of Selaginella, and, like it, producing a 

 number of archegonia. The megaspore, however, 

 is never set free, but remains permanently within 

 the ovule, and this necessitates a quite different 

 method of fertilization (Fig. 14, C). 



The microsporangia of the pine are also in pairs, 

 but are upon the lower side of the sporophyll or 

 scale which bears them. The development of the 

 microsporangium follows very closely that of Se- 

 laginella, and the spores are formed in groups of 

 four, as in all the archegoniates. These micro- 

 spores, or "pollen-spores" (Fig. 14, E), give rise 

 to a rudimentary male gametophyte with two 

 sperms, which are, however, destitute of cilia. 

 When the ripe pollen-spores fall upon the apex of 

 the ovule they germinate, sending out a slender tube 

 which pushes its way through the tissues overlying 

 the apex of the megaspore, and the two sperms, or 

 generative nuclei, pass into the pollen-tube and are 

 thus conveyed to the archegonium. The develop- 

 ment of the pollen-tube does away with the neces- 

 sity of water for effecting fertilization, and the last 

 evidence of the aquatic origin of these plants dis- 

 appears. 



Motile Sperms in Seed-plants. One of the most 

 important discoveries of recent years is the fact that 

 in a number of the lowest seed-plants fertilization 



