224 Plant Life and Evolution 



and this parthenogenesis may be the result of 

 changed conditions; for example, increase of tem- 

 perature, or cultivation in a special nutritive medium. 



Resting Spores. Very commonly the zygote re- 

 sulting from the union of the gametes is a resting 

 cell, or spore, which is adapted to resist the desicca- 

 tion to which fresh-water algae are so frequently 

 exposed. It may be assumed that the various forms 

 of thick-walled resting cells, developed by fresh- 

 water algae, are adaptations to drying up, as these 

 are very seldom met with in the marine algae, 

 where drying up, of course, never occurs. The con- 

 nection of these resting spores with the origin of 

 the lower land plants has already been pointed out. 



Spore Distribution by Insects.' It is in connec- 

 tion with reproduction that plants have been most 

 profoundly influenced in their structures through 

 association with animals. Among the lower plants 

 this is far less marked than is the case among the 

 flowering plants, but there are a number of cases 

 of apparent adaptation to animal agencies in spore 

 distribution. Of course the accidental distribution 

 of spores and other minute germs, which adhere 

 to the bodies of animals, must constantly occur, but 

 there are several cases where this seems to be es- 

 pecially provided for. Thus in the fungus which 

 causes the disease known as Ergot upon rye, at a 

 certain stage in the development of the fungus there 

 is produced a sweet substance which attracts in- 

 sects, to which the spores adhere and are presuma- 



