26 PLANT-LIFE ON LAND [ch. 



would support this hypothesis ; for in these plants 

 such fusions are absent, while the plant itself is 

 here of the nature of a zoospore. It thus appears 

 probable that the motile zoospore condition was 

 prior to such cell-fusions as the gametes undergo. 



Thirdly, a comparison of Ulva or Ulotlwix with 

 other Green Algae, shows that inequality of size of 

 the gameteSj and ultimately marked differentiation 

 of the sexes, may appear. Such a progression from 

 equality to inequality of the gametes is seen also in 

 various types of the Brown Algae. From such facts 

 it may be concluded that the differentiation of sex 

 was acquired subsequently to the initiation of the 

 process of fusion, and that it was attained in several 

 different lines of descent. Its cause is probably to be 

 found in the advantage which a larger store of 

 nutriment brings to the germ. One type of gamete, 

 the male, retained its motility and approximately its 

 original size : the other, the female, became enlarged 

 and receptive, and ultimately it sacrificed its motility 

 to the advantage of better nourishment of the young 

 individual on germination. 



It would be impossible here to do more than 

 suggest such views. The full discussion of them 

 must be left to larger works. But it will be apparent 

 that a study of the common Ulva and its related 

 forms, together with a comparison of other Algae 

 higher and lower in the scale, raises questions which 



