ALTERNATION OF GENERATIONS 145 



" The Archegoniate series," he says, " is undoubtedly 

 of Algal origin, and this their gametophytes amply bear 

 out. . . . They probably sprang from filamentous green 

 aquatic forms, inhabiting shallow fresh water or the 

 higher levels between the marine tide-marks. Certain 

 forms spread to the* land where access of water was only 

 an occasional occurrence ; in these the sexual process 

 could only be effected at time of rains, of floods, or copious 

 dews, and even this might not take place unless the 

 sexual organs were fully mature ; thus less dependence 

 could be placed upon sexuality for propagation, and an 

 alternative method of increase of individuals had to be 

 substituted. This was done by the production of the 

 sporophyte from the zygote ; once fertilised, a zygote 

 might in these plants divide up into a number of portions 

 (carpospores), each of which would then serve as a starting 

 point for a new individual, and dry circumstances under 

 which they would be powdery would favour their dis- 

 persion. In proportion as these plants spread to higher 

 and drier levels (in accordance with the advantage they 

 gained from escape from competition and more free 

 exposure to light for assimilation) the chance of a frequent 

 recurrence of the circumstances necessary for sexual 

 reproduction would be diminished and the dependence 

 on the carpospores for propagation would increase ; 

 consequently the number of spores produced by each 

 sexually-formed sporophyte must be greater, if the race 

 is to survive and be in a position to compete. Any 

 increase in the number of spores entails greater supply 

 of external nourishment during their formation ; this, 

 in the phylum of the Bryophytes, is chiefly supplied from 

 the gametophyte, which shows distinct adaptation to 

 subaerial habit, while the means of nutrition on the part 

 of the sporophyte itself are in these plants very limited 

 and the external morphology of it very slight. In other 

 distinct phyla, however, such as the Filicineae, Lyco- 

 podineae, and Equisetineae, the sporophyte itself assumes 



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