68 Plant Life and Evolution 



without union, thus showing but little difference 

 from the non-sexual zoospores. Cases are known 

 also, even among some of the more specialized 

 forms, where the egg will develop into a new plant 

 without fertilization. 



Asexual Algae. Some highly developed algae 

 are, apparently, entirely destitute of any sexual re- 

 production. Until recently it was supposed that the 

 giant kelps reproduced themselves by non-sexual 

 zoospores only, but it has been found that the sup- 

 posed zoospores are sometimes, at least, gametes. 

 So also the curious siphoneous alga Caulerpa, 

 so far as is now known, multiplies only by the sep- 

 aration of a portion of the plant. 



In the fresh-water green algae the fertilized egg 

 usually develops into a thick-walled " spore " or 

 zygote. This is capable of enduring long periods 

 of drought and is presumably a provision for carry- 

 ing the plant through unfavorable conditions, espe- 

 cially drought or cold. These resting spores almost 

 never are formed in marine plants, as the latter 

 never are subject to prolonged periods of desicca- 

 tion. The zygote on germination very often gives 

 rise to a number of zoospores, by a division of its 

 protoplasm, and thus a new generation starts at once 

 with several individuals. 



In the red seaweeds there is a marked difference in 

 the results of fertilization from that found in the 

 green or brown algae. Instead of the female cell 

 developing at once into a resting spore, it is at once 



