CHAPTER III 



THE PROFOUND EFFECT ON THE STRUCTURE OF PLANTS 

 PRODUCED BY THE NEED FOR EXPOSURE TO LIGHT 



Morphology and Ecology of Leaves and Stems 



N the foregoing chapter we have considered photo- 

 synthesis solely as a physiological process operating 

 within the body of the plant, and have taken no thought 

 for any relations it may have with the world outside. 

 Yet the internal process is dependent on the external world in 

 this very fundamental particular, that the supply of the indis- 

 pensable light, carbon dioxide and water has to come from out- 

 side. Furthermore, and this is a point of importance, the en- 

 vironment rarely offers these essentials in precisely the right 

 quantities, but sometimes too abundantly, oftener too sparsely, 

 and sometimes in ways involving grave dangers. Their photo- 

 synthetic needs plants cannot help, and their environmental 

 conditions they cannot change, but there is one thing that is al- 

 terable, and that is their own structure, with its large poten- 

 tialities of adaptive development. Accordingly, in the course of 

 long ages of slow evolution, plants have become so molded in 

 form and in structure as to bring the photosynthetic process 

 into advantageous or adaptive relation with the conditions of 

 supply of the photosynthetic essentials outside, and in such man- 

 ner, moreover, as to permit of particular adjustment to special 

 peculiarities of the surroundings. Plants are like housekeepers 

 who possess certain needs, and a desire for having the best, but 

 who have no control over the purse-strings ; under the circumstances 

 there is nothing for them to do but adjust the scale and style of 



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