INFLUENCE OF ENVIRONMENT 265 



These resting-spores are usually produced, as in GEclo- 

 gonium or Spirogyra, at the end of the plant's exist- 

 ence, after which the vegetative cells die, leaving the 

 thick-walled resting-spores to carry the plant over to 

 the next growing season. 



These fresh-water plants are, as a rule, far more 

 resistant to changes of temperature than their marine 

 relatives, which frequently are killed very quickly by 

 a slight rise in temperature in the water, this being 

 especially marked in the deep-water red algae, which 

 are only adapted to an environment where the tem- 

 perature remains almost constant and where they are 

 protected from strong illumination. This great sensi- 

 tiveness makes the cultivation in aquaria of most 

 marine algse exceedingly difficult. 



The origin of the first terrestrial plants was due, 

 probably, to the survival of some algal form, which, 

 instead of dying as soon as the spores were ripe, con- 

 tinued to vegetate upon the mud after the subsidence 

 of the water, as is still the case in a few algae. Some 

 of the lower liverworts, which probably resemble more 

 nearly than any existing forms these primitive terres- 

 trial plants, still show this amphibious habit, floating 

 in the water during most of their life, but finally com- 

 pleting their development upon the mud left by the 

 evaporation of the water. The capability of growing 

 with a diminished water supply is an obvious advan- 

 tage, and this is shown by the rapid evolution of these 

 land plants which has resulted in an immense number 

 of most diverse types. 



The mosses, which are doubtless descended from 

 aquatic ancestors, in adapting themselves to their new 



