THE PLANT BODY 



43 



which arise at regular intervals determined by the mathematical 

 spiral arrangement of the leaves and buds, are diageotropic and 

 diaphototropic and thus assume a horizontal position. 



The influence of light in modifying the attitude of the pine 

 tree and its branches is also well illustrated in the figure. At 

 the apex of the tree the young branches assume a more or 



less uniform upward 



direction, due to the 

 combined effect of 

 gravity and light. 

 On the lower por- 

 tions of the trunk the 

 branches are diageo- 

 tropic and nearly hor- 

 izontal near the tree 

 trunk, but farther 

 out toward the apex 

 they curve rather 

 sharply upward, be- 

 coming nearly pro- 

 phototropic at the 

 extreme end of the 

 branch. Careful in- 

 spection of such a 

 tree will demonstrate 



Note the mixed response of the branches to light and 

 the fact that these gravity. Consult the text for a discussion of this figure 



various attitudes as- 

 sumed by the branches of the pine, due to adjusting movements 

 in the response to light and gravity stimuli, secure the best 

 possible exposure of the terminal tufts of needle leaves to 

 light. In a similar manner the ripening seeds in the pine cones 

 are exposed to air currents for dissemination. 



Exactly similar phenomena may be observed in young and 

 adult trees of the spreading type (Fig. 25), where the ultimate 

 form of the branches and the placing of the leaves are deter- 

 mined by responses to light and gravity acting as stimuli, 

 The form of all trees and the proper exposure of their leafage 



FIG. 25. The form and position of the branches of 

 the American elm 



