190 Plant Life and Evolution 



and complexity with which none of the fresh-water 

 algae can compare. This culminates in the giant 

 kelps, whose great leafy shoots may be hundreds of 

 feet in length and in their form suggest the higher 

 land plants. That the increase of salinity seems to 

 induce variation has been noted repeatedly. It has 

 been observed that in adjacent areas, differing 

 merely in their salinity, the less saline water is very 

 much poorer in the number of species than the more 

 saline water. While the brown and red algae pre- 

 dominate in the sea, there are many green algse 

 found there also, and the latter are much less sensi- 

 tive to changes in salinity of water than are the 

 more highly specialized brown and red seaweeds, 

 the latter of which are often quickly killed by slight 

 changes in temperature and salinity. A good many 

 species of both brown and red algse show a cer- 

 tain amount of adaptability, and may adjust them- 

 selves to slightly saline, and even brackish water, as, 

 for instance, near the mouths of rivers flowing into 

 the sea. 



Marine Life Checks Sexual Reproduction. It 

 may be safely assumed that the green seaweeds are 

 probably immigrants from fresh water, which have 

 become modified more or less by their changed en- 

 vironment. A striking peculiarity of all of the 

 green seaweeds is the primitive condition of their 

 reproductive parts, although many of them are 

 plants of considerable complexity. No green sea- 

 weeds are known in which the gametes are perfectly 



