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 have to consider the third group of these 
low plants, the Volvocinee, 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 Protococcacee arose, the firm 
cellulose investment of the cell, found in most plants, 
precluding the active movements typical of the lower 
animals and of the Volvocinee. This stationary green 
cell, with its definite cellmembrane, may be properly 
considered as the starting-point for the series of Green 
Alge, or Chlorophycez, which in their turn are the pro- 
genitors of the much more perfect green land plants. 
The two series, the Volvocinee and Protococcacee, 
may be looked upon as offshoots of a common an- 
cestral type, probably resembling the existing uni- 
cellular Volvocinez. In one direction development 
has proceeded without loss of motion in the cells, re- 
