62 PLANT-LIFE 



lated spermatozoids appears ; these male elements, which 

 are highly specialized, number from 64 to 128. At the 

 proper moment they escape from their parent cell and 

 swarm round the egg-cells. One spermatozoid, more 

 fortunate than its brethren, fuses with an egg-cell, 

 which is thus fertilized. After fertilization the egg- cell, 

 now a zygote, secretes two membranes about itself for 

 its protection; it also changes from green to yellow, 

 and rests for a considerable time before germination. 

 The zygote is a convenient resting-stage, enabling Volvox 

 to tide over the uncongenial winter months. A single 

 adult colony is developed from each zygote, and even 

 before the embryo leaves the parent cell there is some 

 evidence of certain cells being marked off for asexual 

 reproduction. Several asexual generations may occur 

 prior to the appearance of colonies bearing sexual cells. 

 Thus the small but majestic Volvox displays remark- 

 able development. The great majority of its cells are 

 purely vegetative, and only a few are marked off either 

 for asexual or sexual reproduction. Asexual increase 

 occurs for a few generations under congenial conditions, 

 while the sex activity is preliminary to a resting-stage 

 when the conditions are anything but congenial to so 

 delicate a plant. We may opine that constant asexual 

 reproduction would tend to exhaustion, and that the 

 occurrence of an occasional sexual generation leads to 

 the stimulation and renewal of vitality. The egg-cell 

 is relatively large, wellnourished, and passive ; the male 

 elements are highly specialized, active, and seek out the 

 egg-cells. The spermatozoids are produced in goodly 

 numbers, and as only one fuses with an egg-cell, many 

 of them are wasted. This, however, is an instance of 



