308 REVIEWS. 



hibits three kiuds of movement : first that of circumnutatlon, 

 in which, endeavoring to bend in all directions, its tip '* will 

 press on all sides, and thus be able to discriminate between 

 the harder and softer adjoining surfaces, . . . and to bend 

 from the harder soil and follow the lines of the least resist- 

 ance," so modifying advantageously its course from that to 

 which geotropism constantly tends to give it. Moreover, the 

 growing end of the root is sensitive to contact, and in a com- 

 plex manner. If pressed above the tip, it bends there toward 

 or around the impinging bod}^ much as the end of a tendril 

 bends around a su})port : thus it may follow, as roots do, along 

 the unequal surface of a solid body. But, thirdly, if the tip 

 itself be locally pressed, it exhibits different and more sur- 

 prising sensitiveness, for it transmits an influence to an upper 

 adjoining part, causing it to bend away from the affected side. 

 This sensitiveness to contact is confined to a little more than 

 one millimeter of the tip ; the part which bends is G or 7 or 

 even 12 millimeters above. So, when the sensitive tip in its 

 downward growth strikes obliquely upon a stone or other ob- 

 stacle, the part above at this distance, to which some influence 

 must be transmitted, bends and carries the point away from 

 the obstacle. Yet later, when a new portion of the side im- 

 pinges upon the stone or other body, it will bend at that part 

 toward instead of away from it, and so follow along its sur- 

 face. It is the tip, likewise, that can discern that the air is 

 moister on one side than on the other, and which thence " trans- 

 mits an influence to an upper adjoining part, which bends 

 toward the source of moisture." It is the tip only which is 

 sensitive to gravitation. Well may Mr. Darwin affirm that 

 there is no structure in plants more wonderful, as far as its 

 functions are concerned, than the tip of the radicle. Also, 

 that "it is impossible not to be struck with the resemblance 

 between the foregoing movements of plants and many of the 

 actions performed unconsciously by the lower animals." " But 

 the most striking resemblance is the localization of their sen- 

 sitiveness and the transmission of an influence to an excited 

 part which consequently moves. Yet jdants do not of course 

 possess nerves or a central nervous sj'stcm ; and wc may infer 



