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 Volvocinese, i.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, 



