410 VEGETABLE PHYSIOLOGY 



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 

 much less than that exhibited by an animal ; but this, as 

 we have seen, depends upon the differences in the motor 

 mechanisms. In the vegetable protoplasm we have a 

 much slower response, as well as one of a different 

 kind, the effects taking as a rule longer before they are 

 fully manifested and lasting for a longer time after the 

 stimulus has been withdrawn. We have, however, as in 

 the animal mechanism, a much better response to a cumu- 

 lative or prolonged stimulation than to one which is rapid 

 and transitory. 



