THE NERVOUS MECHANISM OF PLANTS 419 
in some alteration of the protoplasm which we cannot 
explain, but there is no evidence of purpose in their origina- 
tion. 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 ana- 
tomical character in any of the sense-organs of a plant, 
there is nevertheless a differentiation of a physiological 
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 
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has been withdrawn.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 cumulative or 
prolonged stimulation than to one which is rapid and 
transitory. 
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