THE NEEVOUS MECHANISM OF PLANTS 409 



differentiation. This we have seen is particularly notice- 

 able with regard to its co-ordinating power. Another 

 feature must be mentioned however before leaving the 

 subject. We do not find/in the plant any indication of 

 anything corresponding to the higher functions of the 

 nervous system of the higher animals. There is no evi- 

 dence of anything which we may compare to consciousness 

 or volition. Though many of the responses to stimulation 

 are eminently purposeful we cannot regard them as in any 

 way modified or held in check by any controlling power. 

 A stimulus will produce its due effect although the mani- 

 festation of that effect at the particular moment may be 

 followed by injurious consequences. The connection between 

 the sense-organ and the motor mechanism is apparently 

 a direct one, and there is no power to modify it possessed 

 by the organism. 



Nor, so far as we know, have we in plants any power 

 of initiative. True, there are many movements and 

 changes which are set up by causes that have their origin 

 in some alteration of the protoplasm which we cannot 

 explain, but there is no evidence of purpose in their 

 origination. Even the locomotion of the Myxomycetes 

 and the Diatoms shows no definite purpose except when 

 it is clearly set up in response to some external stimulus. 



Though there is no particular differentiation of an 

 anatomical character in any of the sense-organs of a 

 plant, there is nevertheless a differentiation of a physio- 

 logical nature in the direction of sensitiveness which will 

 equal if not surpass the powers of the sense-organs of an 

 animal. The tendril of Passiflora appreciates and responds 

 to a pressure which cannot be detected by even the human 

 tongue ; the seedlings of Phalaris readily obey the stimulus 

 of an amount of light which is hardly perceptible by the 

 human eye. Many plants readily detect and respond to 

 the ultra-violet rays of the spectrum, which are utterly 

 invisible to man. 



The extent of the response to any stimulus is of course 



