Environment and Adaptation 189 



their tough leathery fronds. This power of re- 

 taining water is exhibited in a lesser degree by some 

 fresh-water algae, especially those forms which live 

 in the temperate regions and are only part of the 

 time in the water. 



Owing to their perfect adaptation to life in salt 

 water, the brown and red algse have found few com- 

 petitors and may be said to dominate the flora of 

 the sea. Whatever may be the reason, salt water 

 seems to exercise a stimulus which induces a much 

 more luxuriant vegetation, and as we have seen pro- 

 duces greater variation than does fresh water. Sea- 

 weeds growing in pure sea water have been found 

 to be more robust than the same species growing 

 in brackish water, for instance near the mouths of 

 streams. The common rock- weed (Fitcns vesicu- 

 losus) and one of the red algae, a species of Poly- 

 siphonia, are examples. 



Marine Algae. The red algae are not so ex- 

 clusively marine in their habit, and the lower mem- 

 bers of the class show sufficient points of resem- 

 blance to the green algae to make it possible that 

 they may be offshoots of the latter. The conditions 

 of life in the sea are very different from those in 

 fresh water, perhaps the greatest difference being 

 the marked salinity, and consequently greater density 

 of sea water a condition which has involved great 

 changes in the structures of marine plants. The 

 conditions in the sea are evidently conducive to great 

 variability, and we find the seaweeds reaching a size 



