Environment and Adaptation 205 



in deeper water, but it has been suggested that 

 those living in deep water represent a secondary 

 adaptation which has not caused the loss of the 

 pigment, which, however, is no longer essential. 



The case of the red algae seems to be somewhat 

 different. They are, as a rule, shade-loving plants, 

 and grow either in deep water or in the shade of 

 other large algae or of rocks. It has been assumed 

 that the red pigments of these forms enable them 

 to absorb certain light rays which they otherwise 

 could not utilize. How far this is connected with 

 their living in deep water, which absorbs much of 

 the red and yellow rays which are usually essential 

 to photosynthesis in the green plants, has not been 

 satisfactorily demonstrated. 



The influence of light in affecting the form of 

 the higher plants may be readily shown by experi- 

 ment. The difference in habit between plants 

 grown in dense shade and the bright sunshine is 

 very marked, and while other factors than light are 

 undoubtedly concerned, the light relation is one of 

 the most potent factors in the change of form. 

 The influence of light in determining the direction 

 of growth of plants is familiar to every one, most 

 plants growing towards the light, and in the lower 

 plants, as well as the higher, this can be shown. In 

 the flat gametophyte of the ferns and liverworts, 

 the direction of the light striking it determines which 

 is to be the upper and which is to be the lower side, 

 and it has been recently shown by Peirce that in 



