50 EVOLUTION OF PLANTS 



these differences there are based several well-defined 

 orders. 



Attention has already been called to the probable 

 origin of the higher green algse from the Volvocinece, 

 and we have seen how, by the loss of free locomotion, 

 the latter gave rise to the simpler Protococcacese, which, 

 however, give an indication of their origin from motile 

 ancestors by the frequent reversion to the primitive 

 free-swimming condition in their reproductive cells. 



Tracing up the line of ascent in the green algae a 

 step further, there is found a group of forms which 

 consist of rows of perfectly uniform cells, all alike and 

 individually closely resembling in structure the unicel- 

 lular Protococcacese. Sometimes, among the simpler 

 forms of filamentous algge, it is not uncommon to have 

 the filaments break up into separate cells, giving rise 

 to colonies of unicellular individuals which are not to 

 be distinguished from true Protococcaceee. It -is easy 

 to see how the latter, by the repeated division of a cell 

 in a single plant, without separation of the daughter- 

 cells, could give rise to a simple cell-row or filament 

 such as really makes up the plant body in many Chloro- 

 phycese. Indeed, if we follow the life-history of some 

 of these, we find that its individual development fol- 

 lows very closely what we may suppose has been the 

 history of the whole group. Thus the non-sexual re- 

 productive cells are very commonly free-swimming cells 

 (zoospores), which show exactly the structure of the 

 lower Volvocinese (Fig. 8, C). Such zoospores are 

 often biciliate, possess a single chromatophore, eye- 

 spot, and contractile vacuoles* and are veiy sensitive 

 to light, collecting quickly on the lighted side of the 



