THE LIFE OF THE PLANT 



distant part. 1 Moreover, the lower epidermis itself 

 will grow more quickly than the upper, and consequently 

 will more easily yield to stretching. This explanation 

 is also supported by the fact that upward curvature 

 in a lying stem only takes place in the part which 

 grows most rapidly ; in parts where growth has already 

 stopped such a phenomenon is impossible. Therefore 

 by placing the stem in a horizontal position we cause 

 in it unequal and unsymmetrical growth ; the lower 

 side gets ahead of the upper, the stem curves and rises. 

 But this method of argument may appear unconvincing ; 



in that case it can 

 be enforced by a 

 direct experiment. 

 Let us take two 

 similar stems, leav- 

 ing the one to 

 grow vertically and 

 obliging the other to 

 grow horizontally by 

 pushing it through 

 a narrow glass tube 

 where it will not 

 be able to curve. 

 After a certain time 

 let us split the 

 ^'latter stem into an 

 upper and a lower 

 half. The moment 



FIG. 54. we do so the upper 



part will shorten 



and the lower elongate, and if we compare them with 

 the length of a vertical stem we shall see that the 



1 It is quite clear that the resistance exerted by the upper half of the 

 epidermis against the tendency to stretch will be stronger because it acts 

 so to speak on the longer arm of the lever, whereas the lower acts on its 

 shorter arm (see fig. 54, II., c'). 



