ii] HABITS OF COXVOLUTA 39 



observations Charles and Francis Darwin (1880) 

 concluded that the power of gravi-perception is 

 localised in the root-tip. Nor has subsequent criticism 

 succeeded in invalidating this conclusion. 



Now, since the growth-curvature, which results 

 from the perception by the root-tip of the stimulus 

 of gravity, occurs in a region of the root the elon- 

 gating region which is separated from the tip by 

 a region which is not increasing in length, it follows 

 that perception by the root-tip results in an excitation 

 of the living substance of its tissues, and that this 

 excitation gives rise to some change in the tissues 

 which intervene between the perceptive and motor 

 regions. This change, of unknown nature, we may 

 call a nervous impulse, and we may say that, as the 

 result of excitation consequent to perception, a 

 nervous impulse is transmitted from the root-tip to 

 the motor region. Of the nature of this impulse we 

 know nothing ; nor, for the matter of that, is any- 

 thing definite known of the nature of any nervous 

 impulse ; for example, that which travels along a 

 nerve to a muscle in one of the higher animals. The 

 " nervous impulse " may well be of chemical nature, 

 and transmission of such an impulse through living 

 tissues does not connote definite specialised nerves. 

 It is as much the property of protoplasm to transmit 

 nervous impulses as it is of fire to burn, or of a lit 

 fuse to explode a charge of gunpowder. Protoplasm 



