40 EVOLUTION OF PLANTS 
always at the end which is directed toward the light 
when they are in motion. Moreover, in the multicel- 
lular forms (Fig. 6, A) it is those cells which are in 
the forward part of the colony which have the eye-spot 
best developed. 
The multiplication of the lower Volvocinee, ¢.e. the 
unicellular forms, is accomplished by an internal divi- 
sion of the cell-contents after the withdrawal of the 
cilia and the development of a firm cell-membrane. 
This division is accompanied by a preliminary division 
of the nucleus and chromatophore, but the eye-spot and 
contractile vacuoles are probably formed anew in the 
daughter-cells. The latter escape from the mother- 
cell and, developing cilia, become at once complete 
individuals. 
In the higher members of the group, like Pleodorina 
(Fig. 6, A), the plant is multicellular, and the new 
individual arises by the repeated fission of the mother- 
cell, but the resulting cells remain connected, and form 
a multicellular complex of definite form, each cell of 
which has the structure of the simpler unicellular 
forms. In some of these multicellular genera the cells 
are all alike, and are at the same time vegetative and 
reproductive, any cell having the power of dividing 
repeatedly and thus giving rise to a new plant. In the 
most specialized forms in the group, such as Pleodorina, 
each individual has cells of two kinds, small, purely 
vegetative ones and large, reproductive ones. In the 
genus Volvox only a small number of cells have the 
power of dividing, and these have completely lost 
the cilia and eye-spot. Even in the largest specimens, 
where the vegetative cells number several thousand, 
