Factors in Evolution 35 



displaying a large surface of green tissue to the 

 action of light; in other algae there may be dense 

 tufts of small branches whose cells have large 

 chromatophores. In both of these cases there is 

 evidently an increase in the number of green cells 



FIG. 2 



Development of similar leaf-like photosynthetic organs in 

 two quite unrelated plants. 

 A A moss (Tetraphis). 

 B An alga (Sargassum). 



with a corresponding increase in the capacity for 

 carbon assimilation. 



In some of the highly organized seaweeds, like 

 the kelps, there are definite leaf-like organs at- 

 tached to the stem or axis of the plant. This dif- 

 ferentiation of stem and leaves resembles to a re- 

 markable degree, externally, the corresponding 



