G YRA TION OF PLA NTS. 1 57 



on one side with dry nitrate of silver. The injury 

 caused by the caustic was permanent, and the tips 

 turned away in the opposite direction usually with 

 more certainty than when objects were attached. 



For the proper observation of these movements 

 the radicle should be developed at the normal rate. 

 If subjected to high temperature, so that it grow s 

 rapidly, or checked so as to germinate slowly, as in 

 winter, the irritability is much less pronounced. The 

 common garden pea, treated in the same manner, 

 was found to be even more sensitive than the bean. 



A condition of irritability, or sensibility, had been 

 previously demonstrated in radicles, but this always 

 took place higher up, and not at the tip. In such 

 cases the radicle always turns towards the touching 

 object, and not away from it. This movement is of 

 a kindred nature to that exhibited by tendrils, which 

 turn towards and embrace the touching object. But 

 the sensibility of the /// seems to be of quite a 

 distinct character, and one which would prove of 

 considerable service to the plant. As Mr. Darwin 

 says : " The direction which the apex takes at each 

 successive period of the growth of a root ultimately 

 determines its whole course ; it is, therefore, highly 

 important that the apex should pursue from the first 

 the most advantageous direction ; and we can thus 

 understand why sensitiveness to geotropism, to con- 

 tact, and to moisture, all reside in the tip, and why 



