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 alge from the Volvocinee, 
and we have seen how, by the loss of free locomotion, 
the latter gave rise to the simpler Protococcacez, 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 alge 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 uniccl- 
lular Protococcacee. Sometimes, among the simpler 
forms of filamentous alge, 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 Protococcacee. 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- 
phycez. 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 
(zoéspores), which show exactly the structure of the 
lower Volvocinee (Fig. 8, C). Such zodspores are 
often biciliate, possess a single chromatophore, eye- 
spot, and contractile vacuoles, and are very sensitive 
to light, collecting quickly on the lighted side of the 
