264 EVOLUTION OF PLANTS 



which bring them near the surface of the water where 

 they may be exposed to the light. 



One of the most important differences between fresh- 

 water and marine algse, resulting from the nature of 

 their environment, is the different character of some of 

 the reproductive parts. Owing to the constant level 

 of the ocean, aside from the periodic fluctuation of the 

 tides, marine plants are never exposed to the complete 

 desiccation to which nearly all fresh-water plants are 

 at times liable ; nor is there nearly so much difference 

 of temperature in the water at different seasons, as in 

 the shallower and usually variable body of water in 

 most lakes and rivers. We find, therefore, that the 

 marine algse do not develop resting-spores except in 

 rare instances, but the spores are thin-walled or naked 

 cells which germinate as soon as they are mature. 

 Where the plants show a definite periodicity in their 

 growth, as not infrequently occurs in the colder 

 northern waters, the plant is usually perennial by 

 means of a sort of root-stock, or rhizome, from which 

 the annual shoots are produced. 



Most fresh-water algse, however, are plants of very 

 limited growth, and are usually destroyed either by 

 freezing or drying up at the end of their growing 

 season. In the great majority of these are produced 

 special reproductive bodies, usually resting-spores, 

 which are capable of resisting extremes of tempera- 

 ture and dryness which would quickly destroy the 

 actively vegetating plant. These resting-spores are 

 commonly the result of fertilization, but not infre- 

 quently they may form non-sexually, as we find in 

 various of the fission algse, like Nostoc or Anabaena. 



