62 Plant Life and Evolution 



them near the surface of the water, where they may 

 have proper exposure to the light. In these great 

 plants there is also internal differentiation shown, 

 especially in the occurrence of elongated elements, 

 which recall the conducting tissues of the higher 

 plants, and presumably serve the same purpose. 



There is no reason to suppose that the similarities, 

 either of external form or internal structure, point 

 to any near relationship between the brown sea- 

 weeds and any land plants; These resemblances, 

 however, illustrate in a most striking way the forma- 

 tion, in two widely separate groups of organisms, 

 of very similar structures in response to similar 

 needs. It is, in short, a case of " analogy " com- 

 parable to the formation of functionally similar, 

 and structurally different, organs in animals; as, 

 for instance, the wings of birds and insects. 



Reproduction of Brown Algae. Some of the 

 brown algse are purely asexual in their reproduc- 

 tion, while others, like the rock-weed (Fucus), have 

 perfectly developed sexual cells. A remarkable fact 

 is that the forms which are the largest, and struc- 

 turally the most complete, i.e., the giant kelps, are, 

 so far as is known, propagated mainly by non- 

 sexual zoospores. 



Brown Algae Probably Not Related to the Green 

 Algae. The origin of the brown algae is by no 

 means certain. At present the available evidence 

 seems to indicate that they are not directly related to 

 any of the green algae, but constitute a quite inde- 



