402 VEGETABLE PHYSIOLOGY 



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 com- 

 parable to the property of responding to the attraction of 

 gravitation, while the chemotactic behaviour of the organ- 

 isms described in the last chapter suggests a rudimentary 

 power of taste or smell, or both. 



The differentiation of these mechanisms in plants is 

 anatomically very slight. Indeed, no dissection will ex- 

 hibit 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 vege- 

 table protoplasm is not anatomical. But such a differentia- 

 tion is very considerable physiologically. The degree of 

 sensitiveness which many of these organs possess is ex- 

 treme, 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. 



