160 EVOLUTION OF PLANTS 



known as a seed. This peculiar modification of the 

 macrosporangium to form a seed is the real distinguish- 

 ing characteristic of the Spermatophytes. 



The microspores or pollen-spores of the seed plants 

 differ very little from those of the ferns either in form 

 or development, and indeed are strictly homologous with 

 the spores of all Archegoniates, where, as we have seen, 

 the spores invariably arise from the division of the 

 sporogenous cell into four equal parts. 



Owing to the position of the archegonium within the 

 macrosporangium, the method of fertilization is different 

 from that in the Pteridophytes, where the free gameto- 

 phytes are directly exposed to the action of water, and 

 motile spermatozoids are produced in the antheridium. 

 The pollen-spore of the Spermatophytes on germination 

 produces a long tubular filament within which is con- 

 tained the very rudimentary antheridium with usually 

 two sperm-cells. In its growth the pollen-tube grows 

 down through the tissues above the apex of the female 

 gametophyte, and finally reaches the archegonium, where 

 it discharges the sperm-cells, one of which fuses with 

 the egg-cell, thus effecting fecundation. Until very 

 recently it was supposed that the absence of motile 

 spermatozoids formed an absolute distinction between 

 Pteridophytes and Spermatophytes, but the discovery of 

 large fern-like spermatozoids in certain Cycads, as well 

 as in the curious genus Gingko, has broken down the 

 last barrier between the two groups. 



The " flower " in most Spermatophytes is a collection 

 of sporophylls, or spore-bearing leaves, the carpels, 

 bearing macrosporangia (ovules), and the stamens, bear- 

 ing the microsporangia (pollen-sacs). These sporophylls 



