402 VEGETABLE PHYSIOLOGY 
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 less 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; the muscular sense, or power of appreciating weight, 
is perhaps comparable to the property of responding to the 
attraction of gravitation, 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 visibly with the perception of the 
stimulus. It remains 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 
differences in hygrometric condition. There is nothing 
anatomical corresponding to this. If a sensitive organ is 
acted upon at the same time by two stimuli which both 
affect it, and which usually produce opposite movements, 
the resulting position is always that which would be caused 
by the stronger of the two. The organ is, in fact, able to 
receive both stimulations simultaneously, and to respond to 
each as if the other were not received. 
If we turn to the second feature of the nervous system, 
we find that the motor mechanism of the plant seems at 
first to be entirely different from that of the animal. 
