26 KANSAS CITY REVIEW OF SCIENCE. 



"Authors seem generally to look at the bending of a radicle toward the 

 center of the earth as the direct result of gravitation, which is believed to 

 modify the growth of the upper or lower surfaces in such a manner as to induce 

 curvature in the proper direction. But we now know that it is the tip alone 

 which is acted on, and this part transmits some influence to the adjoining parts, 

 causing them to curve downward. Gravity does not appear 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. 



" A radicle may be compared with a burrowing animal, such as the mole, 

 which wishes to penetrate perpendicularly into the ground. By continually mov- 

 ing his head from side to side, or circumnutating, he will feel any stone or other 

 obstacle, as well as any difference in the hardness of the soil, and he will turn 

 from that side ; if the earth is damper on one than on the other side, he will turn 

 thitherward as a better hunting ground. Nevertheless, after each interruption, 

 guided by a sense of gravity, he will be able to recover his downward course and 

 to burrow to a greater depth." 



In a remarkable degree, the action of the sensitive and almost sensible root-tip 

 is analogous to that of the brain of some of the lowest animals. The circumnutation 

 of leaves is an upward and downward motion, by which are described very narrow 

 ellipses. The " sleep" of the leaves of certain plants — that is, the assumption of 

 a direction at night different from that taken during the daytime — is shown by 

 Mr. Darwin to be a modification of the motion of circumnutation. The leaves, 

 both when awake and when asleep, are continually in motion, the motion being 

 most rapid at the periods of transition between light and darkness. The sleep- 

 ing leaves or leaflets place themselves with the axes more or less nearly vertical, 

 some by turning upward, others by i^turning downward. The purpose of sleep- 

 ing is conclusively shown to be the reduction of the amount of radiation of heat 

 from the leaf — to prevent it from becoming chilled at night. The tender seed- 

 leaves of very many plants, whose leaves do not "sleep," assume this vertical 

 position at night. Mr. Darwin observes : 



"It is impossible not to be struck with the resemblance between the fore- 

 going movements of plants and the actions performed unconsciously by the lower 

 animals. With plants an astonishingly small stimulus suffices ; and even with al- 

 lied plants one may be highly sensitive to the slightest continued pressure and an- 

 other highly sensitive to a slight momentary touch. The habit of moving at cer- 

 tain periods is inherited by both plants and animals; and several other points of 

 similitude have been specified. But the most striking resemblance is the local- 

 ization of their sensitiveness, and the transmission of an influence from the excit- 

 ed part to another which consequently moves. Yet plants do not, of course, 

 possess nerves or a central nervous system ; and we may infer that with animals 

 such structures serve only for the more complete transmission of impressions, and 

 for the more complete intercommunication of the several parts." 



This book is characterized by the same freedom from dogmatism and the 



