The Lower Plants 53 



" Volvocales " are composed of a much smaller 

 number of cells, and some of them are unicellular 

 (see Fig. 3). Under certain conditions, the cell 

 divisions take place with great rapidity and within 

 a few days a small pool or watering trough may 

 have its waters made quite green by the presence 

 of millions of these little plants. 



The structure of the cells in Volvox is like that of 

 most of the lower green plants. Each cell has a 

 single large green chromatophore. There is a dis- 

 tinct nucleus and there is also a peculiar body, red 

 in color, and apparently an organ for light percep- 

 tion, hence often known as the eye spot. These 

 organisms are extremely sensitive to light, and by 

 means of the two cilia with which each cell is 

 provided they swim toward the source of light. 

 This moving toward the light is undoubtedly asso- 

 ciated with the function of photosynthesis. 



New plants arise by division of special cells, and 

 as we have seen, these divisions may follow very 

 rapidly, so that the number of individuals may in- 

 crease enormously in a very short time. Another 

 form of reproduction, however, occurs at the end 

 of the growing season, at which time there are de- 

 veloped special structures fitted to survive the drying 

 up of the water, which is fatal to the plant in its 

 active condition. These structures are the " rest- 

 ing spores," which are provided with a thick cell 

 wall and contain an accumulation of food materials, 

 so that they are thus protected against the effects 



