THE ORIGIN OF NERVES. 281 



the case of the Venus's-flytrap, or in the hairs of the sun- 

 dew leaves the cell-walls appear to influence the rate of 

 transmission of the impulse which brings the irritability of 

 the plants into action. Darwin's researches on " Insecti- 

 vorous Plants " contain much suggestive matter bearing on 

 the present point. The stimulus applied to the leaf-hair of 

 a sensitive-plant can be seen to pass through the cells of 

 the hair, its passage being indicated by the successive 

 movements and contractions of the protoplasm of the cells ; 

 and Darwin remarks that, in the case of the sundew's hairs, 

 the cell-walls appear to present obstacles to the quick pas- 

 sage of the stimulus. This conclusion is fully supported 

 by the fact that a stimulus passes more rapidly in a longi- 

 tudinal than in a transverse direction in the leaf of the 

 sundew; and this for the reason that in the longitudinal 

 pathway through the leaf there are fewer cell-walls than in 

 the other direction. Summing up the question of plant- 

 nervousness, therefore, we may hold that the sensibility of 

 plants is limited chiefly by the fact that their protoplasm 

 is even in the lowest plant organisms enclosed within cells j 

 and that the cell-walls appear to present partitions; which, 

 in the great majority of cases, act as effectual barriers to 

 the quick transmission of impulses. Certain plants, as we 

 have seen, have surmounted the difficulty in a very decided 

 fashion ; but even in their case the sensitiveness is inferior 

 to that of the animal, and their impulses are of slower kind 

 than those of their neighbours in the " regne animal." 



But if the special constitution and structure of the plant 

 militates against the development of nerves within the con- 

 fines of the vegetable world, the conditions of animal life 

 present favourable conditions, on the other hand, for the 

 higher exercise of sensation. There are no obstacles to the 

 free passage of an impulse through the amoeba's body, and 

 special tracts and pathways, named nerves, are developed 

 for the transmission of impulses in animals of by no means a 

 very advanced grade. Hence, the main question at issue is 

 that of accounting for the progressive development of distinct 



