44 MOVEMENTS OF PLANTS 



gravitational sense, and they increase or diminish 

 the angular bend in their stalk till the right position 

 is attained, as shown in Fig. 3. 



All these cases of plants executing certain useful 

 curvatures, which occur when the plant is displaced 

 as regards the vertical, and cease when the habitual, 

 relation is reached, all these, I say, seena to me only 

 explicable on the theory that gravitation does not 

 act as a mechanical influence, but as a signal which 

 the plant may neglect entirely, or, if it notices, may 

 interpret in any way ; that is, it may grow along 

 the indicated line in either direction or across it at 

 any angle. It may be said that this is no explana- 

 tion at all, that it only amounts to saying that the 

 plant can do as it chooses. I have no objection to 

 this, if the meaning of the word 'choice' be defined. 



I am now going to deal with the subject of 

 movement from a somewhat different point of view, 

 namely, with the object of showing that it is possible 

 to discover the part of the plant which reads the 

 signal: and this is not necessarily the part that 

 executes the correlated movement. In the reflex 

 movement of an animal (for instance, a cough pro- 

 duced by a crumb going the wrong way), we dis- 

 tinguish the irritation of the throat and the violent 

 action of the muscles of the chest and abdomen; and 

 further, the nervous machinery by which the stimu- 

 lus is reflected or switched on, by way of the central 

 nervous system, from the throat to the muscles 

 concerned in coughing. In the plant, too, if we are 

 to compare its movements to the reflexes of animals 

 (as has been done by Czapek), we must distinguish 

 a region of percipience, another of motility, and the 



