IRRITABILITY 21 



made of use to the plant by the arrangement 

 and division of its chlorophyll areas, and 

 the morphological necessities will account 

 for the method of differentiation of the 

 shoot, and the very great degree of segmen- 

 tation and branching which it has attain- 

 ed. The segmentation of the shoot has made 

 possible not only the profitable display of 

 ever-increasing areas of chlorophyll-bearing 

 tissues, the proper elevation, orientation and 

 isolation of the reproductive organs, but also 

 a separation of the minor functions and the 

 differentiation of special organs for their per- 

 formance. The separation of nutritive, repro- 

 ductive and other functions has been accom- 

 panied by a contemporaneous separation and 

 development of the special forms of irritabil- 

 ity which are concerned with the forces dealt 

 with by each organ. Thus, for example, the 

 most important factor in the processes carried 

 on by the leaf is the radiant energy derived 

 from the sun. As a necessary concomitant of 

 the advantageous use of this energy, the leaf 

 has developed a strongly marked irritability 

 to light and heat rays. 



By these special powers it is enabled to 

 move its chlorophyll-bearing areas in the 

 leaves into such position that the exact inten- 

 sity of light and heat rays suitable to its 

 specific constitution will be received and as a 



