240 



The Living Plant 



in the second it is most ahundant on the side towards the ger- 

 minator. The experiment, therefore, shows that roots turn in the 

 direction where moisture is most plenty; — that is, they possess a 

 definite hydrotropism, another typical form of irritable response. 

 The advantage of hydrotropism is perfectly evident when one 

 recalls that the very first function of roots is the absorption of 



Fig. 84. — Porous water-filled cylinders, to which seeds of Mustard were attached. That 

 on the left was then kept in a saturated, and that on the right in a drier, atmosphere. 



water. The stimulus acts in this waj^; the water, absorbed 

 more rapidly on the side of its greatest abundance, doubtless 

 causes an osmotic swelling and tension stronger on that side 

 than on the other; and this difference is ample to establish a line 

 of direction towards which the roots turn in their growth. It is 

 equally easy to see why stems and leaves display no hydrotro- 

 pism at all, for, as they do not absorb any water under normal 



