GROWTH 



119 



an advantage to the plant, since 

 the leaves are thereby more 

 rapidly brought into positions 

 of best exposure to air and sun- 

 light. The growth of several 

 internodes at the same time, 

 and their elongation throughout 

 their entire length, carries the 

 tip of the steiii forward with 

 much greater force than if 

 growth were confined to a short 

 zone, as in the case of the grow- 

 ing root. But a more rapid and 

 forceful advance of the root-tip 

 through the soil might result in 

 serious or fatal injury, on ac- 

 count of the resistance and ob- 

 stacles encountered in the soil. 

 Thus the different manner of 

 growth of stems and roots is seen 

 to be of direct advantage to the 

 plant as a whole. 



122. Growtii of Leaves. — In 

 tropical climates leaves that 

 have once begun to form con- 

 tinue to grow until they reach 

 full size; but in temperate cli- 

 mates, having an alteration of 

 summer and winter, this is not 

 the case. Here the leaves of any 

 given season are all formed dur- 

 ing the preceding growing sea- 

 son, and remain over winter 



A 



Fig. 78. — Diagram showing 

 mode of growth in length of a 

 portion of a stem of bindweed 

 {Convolvulus). ^, stem with in- 

 ternodes marked off into inter- 

 vals of I cm.; B, the same stem 

 24 hours later, showing the rela- 

 tive elongation of the various 

 internodes. (Cf. Fig. 77.) (Re- 

 drawn from Bonnier and Leclerc 

 du Sablon.) 



