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stimulus has been withdrawn.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. 
