54 Plant Life and Evolution 



of drought, and the food supply within the cells 

 permits of very rapid germination when the proper 

 time comes. These " resting spores " are the result 

 of a simple sexual process. Two special cells, the 

 " gametes " or sexual cells, fuse into a single one, 

 and the resulting cell, or " zygote," becomes the rest- 

 ing spore. In the unicellular forms, the gametes are 

 alike, both being provided with cilia and hardly dis- 

 tinguishable from the ordinary individuals; but in 

 the higher forms, like Volvox, one of the gametes 

 is a very small and active cell, the male or sperm 

 cell, while the other is very much larger and is desti- 

 tute of cilia. The latter is the female cell egg, or 

 ovum and when fertilized by the sperm, develops 

 into the resting spore. 



Because of their permanently motile condition, 

 and their unmistakable resemblance to the Flagel- 

 lata, the Volvocales are sometimes claimed by zo- 

 ologists as animals; but the cell structure and the 

 reproduction are in all respects like those of the 

 lower algae, and there is no question but that the 

 relationship with the latter is a very close one. 



Among the unicellular Volvocales it is not un- 

 common to find the plants assuming a quiescent con- 

 dition. The cilia are lost, and the cell then closely 

 resembles many of the ordinary unicellular algae. 

 It is highly probable that the lower green algae 

 originated in some such fashion from unicellular 

 Volvocales, this being indicated by the frequent 

 reversion in so many algae to the motile condition 



