ORIGINATION OF SELF-GENERATING MATTER 



285 



original. Now the germination and growth of these asexually pro- 

 duced spores could proceed in the absence of free water, and in 

 ordinary soil in which all of the water present was represented by 

 the hygroscopic layer coating the 

 minute particles of which it is com- 

 posed. 



Even with this development, 

 however, plants could not get very 

 far from the water, since this ele- 

 ment in a free state was still neces- 

 sary for the activities of the gameto- 

 phyte, or sexual generation. The 

 sporophyte, however, continued to 

 increase in size and to wax in 

 importance in the life-cycle of the 

 species, until finally its body was 

 much larger than that of the 

 gametophyte. This feature is well 

 illustrated by the tree ferns in which 

 the sporophyte is a massive plant 

 while the prothallium, or sexual 

 generation, is a small thallose 

 structure only a few millimeters 

 across. 



Eventually, however, the spores 

 formed by the sporophyte, capable 

 of living on dry land, were germi- 

 nated in place, giving rise to sexual 

 individuals, which were also held 

 and nursed in the tissues of the 

 sporophyte. Then in completion 

 of the movement, accessory struc- 

 tures, including the pollen-tube, 

 were formed, by which the sexual 

 reproductive elements might be brought together independently of 

 external conditions. By these steps the seed-plant originated and 

 vegetation became truly and wholly able to occupy the land — a most 



Fig. 2. — The gametophyte, or sexual 

 generation of a fern. Reproduction is 

 accomplished by the movement of a 

 sperm (a) from the antheridium A to 

 the archegonium B where it fuses with 

 the egg, accomplishing fertilization. The 

 sperm swims through a thin film of water 

 which may be present. The absence of 

 the film by aridity is unfavorable to the 

 reproduction and continuation of this 

 type of vegetation. The germination of 

 the egg produces the sporophyte or fern 

 plant ordinarily known (see Fig. 3). 



