44 EVOLUTION OF PLANTS 



and soon assume all the characters of the parent- 

 cell. 



In the more specialized forms, like the water-net 

 (Fig. 7, B, C), which may be said to bear somewhat 

 the same relation to the lower forms that Volvox does to 

 the lower Volvocinese, the young individuals become 

 united into a colony of definite form. The zoospores, 

 or ciliated, reproductive cells, remain within the mother- 

 cell, where they grow together into a small net which 

 is set free by the gradual dissolution of the wall of the 

 mother-cell. The sexual cells, however, are ejected 

 from the mother-cell, and unite two and two into 

 spores which, in time, after several intermediate stages, 

 give rise to several new nets. In nearly all these 

 forms, both non-sexual and sexual cells show a rever- 

 sion to the primitive biciliate cell, resembling closely 

 in its structure the individual cells of the lower 

 Volvocinese from which these forms have presumably 

 originated, but having only a very limited period of 

 independent, active existence. 



Within the Protococcacese we find considerably less 

 specialization than exists among the Volvocinese. Thus 

 none of them can be properly considered as truly multi- 

 cellular, for such forms as the water-net and its allies are 

 really colonies of originally unicellular units, and all 

 the individuals of the colony are alike. So, too, sexual 

 cells, when they exist at all, are of the simplest type, 

 with no difference between the male and female cells 

 or gametes, which closely resemble the non-sexual 

 zoospores. 



As the lower Protococcaceae are very intimately con- 

 nected with the series of green algre which are the un- 



