Chap. L] OF A NERVOUS SYSTEM. 15 



if not comparatively structureless, must be composed of 

 plastic units of living matter, not marked off from one 

 another by definite and lowly vitalized cell- walls. 



The vegetal mode of growth is, therefore, as already 

 indicated, precisely of such a kind as to unfit it in an 

 eminent degree for developing any notable power of appre- 

 ciating varied external impressions and yielding immediate 

 and discriminative responses thereto. 



The nearest approach to such powers and actions in the 

 vegetal world is met with amongst the so-called '' Insec- 

 tivorous Plants," upon whose peculiarities Mr. Darwin 

 has lately given us much information. If we dwell for a 

 few moments upon these highest manifestions of the kind 

 known to occur amongst plants, the reader may the bettor 

 comprehend the great gulf which separates the vegetal 

 from the animal world in regard to their respective powers 

 of discrimination and motor response. 



When the three hair-like projections on the upper surface 

 of the leaf of the Venus fly-trap are touched, they almost 

 instantly communicate a stimulus to the cells on each 

 side of the mid-rib, whereby some change is induced in 

 them, and the two halves of the leaf are made to 

 approach one another. The nature of the change has not 

 yet been fully ascertained, though the evidence adduced 

 by Darwin seems to show that it is, at least in part, due 

 to the contractility of the cells above mentioned. A simi- 

 lar influence appears to be transmitted from the glands 

 that tip the hair-like projections fringing the leaves of the 

 Sun dew, to certain cells near the base of these bodies, 

 whereby motion is produced. In this latter plant, a very 

 appreciable interval occurs between the time of irritation 

 pnd the answering movement. Mr. Darwin has never 

 known the interval to be less than ten seconds, though 

 even in the one case in which it took place so rapidly aa 



