192 Plant Life and Evolution 



kelps, no sexual reproduction has been demon- 

 strated. 



It is pretty clear that the lack of resting spores 

 in the marine algse is correlated with their absolute 

 freedom from danger of drying up. It may be that 

 the advantages of the numerous quickly germinating 

 reproductive cells, such as the zoospores and iso- 

 gametes of most of the brown and green seaweeds, 

 or the spores of the red algae, have been so great 

 that they have remained in their present state of 

 development as the result of natural selection. 



Surf Algae. Certain very obvious adaptations to 

 a marine environment are the tough and flexible 

 tissues found in the larger kelps and red seaweeds 

 which are exposed to violent surf. This is very 

 beautifully shown in the great kelps that abound 

 along the rocky coasts of Pacific North America. 

 Some of them, like the sea-palm (Postelsia) (Fig. 

 20), cling to the most exposed rocks, where they 

 are constantly battered by the full force of the heavy 

 waves that dash against the shore. These kelps, with 

 their elastic leathery fronds and powerful holdfasts, 

 grip the rocks securely and withstand uninjured the 

 heaviest pounding of the surf. 



TERRESTRIAL PLANTS 



The conditions of life in water are much less 

 variable than on land. Temperature changes are less 

 extreme and rapid, and of course the amount of wa- 



