THE NEBVOUS 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. If 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 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 ; 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 



