THE NERVOUS MECHANISM OF PLANTS 411 
There is, however, some doubt as to how far this interpre- 
tation is justified. 
It is impossible to avoid the conclusion that we have to 
do in these instances, which are only representative ones, 
with a localisation of sensitiveness, or the differentiation of 
sense-organs. Jf we compare them with physiologically 
corresponding regions in the animal we find a certain 
agreement, though it must not be pressed too far. The 
power of sight is very complete in the higher animals, 
partly in consequence of the highly differentiated character 
of the eye; but in the lower animals it becomes less and 
less perfect as we descend in the scale, till in some it goes 
probably little further than the power of appreciating light. 
This power we have seen to be possessed by certain parts of 
the young seedlings of various plants in a very high degree, 
and by other organs to a legs extent. The sense of touch 
may be compared with the power of responding to the 
stimulus of contact shown by tendrils and by the tips of 
roots ; while the chemotactic behaviour of the organisms 
described in the last chapter suggests a rudimentary power 
of taste or smell, or both. 
The differentiation of these mechanisms in plants is 
from an anatomical standpoint very slight. Indeed, no 
dissection will exhibit any special feature of the structure 
which can be associated certainly with the perception of 
the stimulus. It is a property of the protoplasm of the 
cells in question, but is only one among many properties 
that the latter possesses. The direction of differentiation 
in vegetable protoplasm is not anatomical; but such a 
differentiation is very considerable physiologically. The 
degree of sensitiveness which many of these organs possess 
is extreme, as we have shown already by several examples. 
Another somewhat remarkable fact, in view of the 
peculiar character of the differentiation of these organs, is 
that the same sense-organ is sensitive to many stimuli, 
though in different degrees. We have noticed in the case 
of the root that its tip appreciates contact, gravitation, and 
