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. 



