The Origin of Land Plants 81 



of life in fresh water are very uniform everywhere, 

 and it is not strange therefore that the range of 

 structure exhibited by the fresh-water green algae 

 is comparatively slight. Owing to the density of 

 the medium in which they live, no mechanical or 

 supporting tissues are required, as they are entirely 

 supported by the water. In consequence most algae 

 when taken from the water collapse. Moreover, 

 owing to their complete submersion there is no loss 

 of water from evaporation, and the cells, therefore, 

 are not protected against evaporation and water is 

 absorbed by all of the superficial cells. 



Fresh- water Plants Require Protection Against 

 Desiccation. However, as most fresh-water plants 

 are liable to be destroyed by the drying up of the 

 temporary ponds or streams in which they live, it is 

 necessary to provide for their survival through 

 periods of drought to which they may be subjected. 

 Marine plants never being exposed to prolonged 

 drying up, although they may be uncovered by low 

 tide for several hours, have no need for such pro- 

 tective devices, and hence, resting spores are rarely 

 found in these marine forms. Some of the lowest 

 plants, like certain of the blue-green algae and the 

 common " Protococcus " forms, may be completely 

 dried up in their vegetative condition, remaining 

 dormant for an indefinite period, and then when 

 water is supplied to them promptly revive and re- 

 sume their activity. Moreover, some of these low 

 forms, as well as others of higher rank, find suf- 



