46 EVOLUTION OF PLANTS 



secondarily derived, cannot be settled, but it is ex- 

 tremely likely that the whole group is a very ancient 

 one and adapted to conditions quite unsuited to ordi- 

 nary types of vegetation. The presence of cilia in many 

 of the bacteria suggests possible affinity with the primi- 

 tive motile forms from which the higher plants have 

 originated. 



Finally we havd to consider the third group of these 

 low plants, the Volvocinese, which, while showing 

 marked similarity to the lower animals in the actively 

 motile vegetative cells, still in the possession of a cellu- 

 lose membrane and definite green chromatophore, as 

 well as in their nutrition and reproduction, are typical 

 plants. Within this group there is considerable differ- 

 entiation of the plant body and the reproductive cells, 

 but it is among the lowest members of the group that 

 we are to look for the point of contact with the higher 

 plants as well as possibly with the lower animals. 



By the loss of active motion we may assume that 

 forms like the lower Protococcacese arose, the firm 

 cellulose investment of the cell, found in most plants, 

 precluding the active movements typical of the lower 

 animals and of the Volvocinese. This stationary green 

 cell, with its definite cell-membrane, may be properly 

 considered as the starting-point for the series of Green 

 Algse, or Chlorophyceae, which in their turn are the pro- 

 genitors of the much more perfect green land plants. 



The two series, the Volvocineae and Protococcacese, 

 may be looked upon as offshoots of a common an- 

 cestral type, probably resembling the existing uni- 

 cellular Volvocinese. In one direction development 

 has proceeded without loss of motion in the cells, re- 



