INFLUENCE OF ENVIRONMENT 265 
These resting-spores are usually produced, as in CEdo- 
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 alge, 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 alge 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 alge. 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 
