IRRITABILITY 243 



spends to the slight stimulus by increasing his speed, but 

 the same horse at the end of a hard day's journey can- 

 not be made by the same stimulus, or even by a greater, 

 to give the same response. This indicates two things : in 

 the first place, that the touch of the whip or the repeated 

 contact with a solid body do not of themselves increase 

 the speed of the horse or accomplish the bending of the 

 tendril, but that they are merely stimuli, that these slight 

 influences set in operation other forces. 



Our conception of the stimulus as merely the feeble force 

 which sets other forces in operation is justified by further 

 consideration of the behavior of horse and tendril. Though 

 the fresh horse promptly responds to the light touch of the 

 whip, there is actually the lapse of some time between 

 stimulus and reaction, but this time is brief because the 

 nerve and muscle systems of the horse are exquisitely 

 adapted to receiving and responding to such stimuli. They 

 are especially differentiated to accomplish a few purposes. 

 The tendril responds more slowly to contact. There is 

 what is termed the "latent period" between stimulation 

 and reaction, ranging from five seconds or less to an hour 

 or more according to the species, but during this period 

 the forces set in operation by the stimulus are working. 



The means by which a tendril curves around its support 

 is not definitely known, and it may well be that the me- 

 chanics of curvature are not the same in all cases. The 

 older plant physiologists, like Sachs* and de Vries,t as- 

 serted that the bending is due to changes in the rate of 

 growth on the two sides, the side in contact with the sup- 

 port growing less rapidly, the opposite side more rapidly, 

 than before the contact was made. MacDougalt dissents 

 from this generally accepted opinion, believing that there 

 are not such changes in growth-rate, but that the side in 

 contact with the solid object contracts decidedly and that 



* Sachs, J. von. Physiology of Plants. Eng. Edition, 1887. 



f De Vries, H. Langenwachsthum der Ober- und Unterseite Ijriimmender 

 Eanken. Arb. d. Bot. Inst. Wiirzburg, Bd. I., 1873. 



t MacDougal, D. T. Mechanism of curvature in tendrils. Annals of Bot- 

 any, X., 1896. 



