12 



LIVING PLANTS 



Movement 

 connected 

 witli growth 



ing the structure of the organ, one could pre- 

 dict the direction of movement under stimu- 

 lation. But we shall have to look beyond 

 and above simply mechanical laws for an ex- 

 planation, for the living organism acquires 

 specific powers of adaptation and heredity. 



Although the force which a plant can exert 

 amounts to several atmospheres, it is only in 

 the young tender portions, usually at the ends 

 of the branches of the stem and root, that 

 this force can be successfully applied to secure 

 movement of the whole organ. It therefore 

 comes about that movement in plants is 

 oftenest associated with growth. This ar- 

 rangement permits each root tip and growing 

 stem to have its own kind and degree of sen- 

 sitiveness. Thus we find by experiment that 

 while the first root which starts from a seed, 

 the tap root, is sensitive to gravity in such a 

 way that it places itself parallel to the direc- 

 tion of the impinging force and points directly 

 downward, the secondary roots, which 

 branch from it, are sensitive after a different 

 fashion, and instead of growing parallel to 

 the force, grow at an angle to it, the exact 

 angle being different for different kinds of 

 plants. The tertiary roots, or next set of 

 branches, are usually very little sensitive to 

 gravity, or if they are sensitive they assume a 

 nearly horizontal position. The stems react 



