184 GENERAL BIOLOGY 



The advantage of asexual reproduction or spore- 

 formation lies in the fact that species may thus tide 

 over periods of unfavorable conditions, or, on the 

 other hand, rapidly multiply the vegetative phase of 

 the plant's life in favorable circumstances. In 

 leaving the water for the land, the original aquatic 

 traits were at first, in large measure, retained; 

 that is, the gametophyte phase was most prominent, 

 and the gametes were aquatic. This is a condition 

 we find in the mosses and liverworts. In the ferns we 

 still find the gametes to be motile, aquatic cells, but 

 the difficulties and dangers of this mode of reproduc- 

 tion are compensated by a marked increase in the 

 degree of specialization which the asexual or spore- 

 forming phase has attained. 1 



Finally, in the seed plants we find the free-swimming 

 germ-cells replaced by gametes that are throughout 

 protected and inclosed by the tissues of the gameto- 

 phyte, and the young sporophyte that results from 

 their conjugation is protected and supplied with 

 food. Division of labor has brought about an in- 

 creasing efficiency from the standpoint of competi- 

 tion with other types. The seed plants, independent 

 of water for the purpose of zygosis, and adapted to 

 secure the greatest protection for the developing 

 sporophyte, as well as for its maximum dispersal, 

 have a very great advantage over the " lower " 



x The advantage of asexual reproduction lies in the fact that the 

 species is much less likely to be exterminated if the vegetative phase 

 of the plant's life is emphasized and a wide dispersal secured. More- 

 over, the formation of spores enables the species to tide over periods of 

 unfavorable conditions. 



