POWER OF MOVEMENT IN PLANTS. 309 



tliat with animals such structures serve only for the more per- 

 fect transmission of impressions and for the more complete 

 intercommunication of the several parts." The closing sen- 

 tence of the book may be appended to this. " It is hardly an 

 exaggeration to say that the tip of the radicle, thus endowed 

 and having the power of directing the movements of the ad- 

 joining parts, acts like the brain of one of the lower animals." 



The movements " excited by light and gravitation," as well 

 as the nyetotropic or sleep-movements so called, are (as we 

 have already stated ) all referred by Mr. Darwin to modified 

 circumnutation, "which is omnipresent while growth lasts, 

 and after growth has ceased whenever pulvini are present," 

 as in several classes of leaves. As respects the relation of 

 external agents to the movements, note Mr. Darwin's re- 

 mark : " When we speak of modified circumnutation we 

 mean that light, or the alternations of light and darkness, 

 gi-avitation, slight pressure or other irritants and certain innate 

 or constitutional states of the plant, do not directly cause 

 the movement ; they merely lead to a temporary increase or 

 diminution of those spontaneous changes in the turgescence 

 of the cells which are already in progress." 



Certain parts of plants turn or grow earthward. When 

 this is attributed to gravitation, as it commonly is, the physi- 

 cists have opportunity to complain of the misuse of a term. 

 Although Mr. Darwin, like other writers, speaks of the in- 

 fluence of light and of gravitation in the same breath, without 

 discrimination, we note with satisfaction his disagreement with 

 those who " look at the bending of a radicle towards the 

 centre of the earth as the direct result of gravitation," and 

 note especially the closing dictum. "Gravity does not ap- 

 pear to act in a more direct manner on a radicle than it does 

 on any lowly organized animal, which moves away when it 

 feels some weight or pressure." Why, we would ask, need 

 the word gravity or gravitation be used at all in this con- 

 nection ? 



The introduction to this volume contains a short article 

 upon the terminology which is adopted in it, chiefly as re- 

 gards such words as epinasty and hyponasty, geotropism and 



