DETECTION OF LATENT STIMULUS 459 



Balance is employed, the ordinary magnification is enough 

 to show, in a very marked manner, all the phases of these 

 transient variations. The records here reproduced have 

 been in fact reduced to one-third of the originals. 



The sensitiveness of the arrangement can be very much 

 exalted by observing the balanced line of light with its devia- 

 tions, through a telescope placed at a distance. In this way, 

 I have been able to detect a variation from the normal rate of 

 growth, of so little as a two millionth part of 1 mm. per second, 

 within so short a period of observation as ten seconds. 



Detection of absorbed stimulus by negative after- 

 effect. — We must now revert to the question of the detection 

 of latent stimulus by an increase in the rate of growth, 

 and we shall first take that simple case in which there is no 

 loss of energy from irreversible effects due to molecular 

 friction. The energy of external stimulus will here find 

 complete expression in doing external and internal work. 

 If the external stimulus remain constant, the sum of these 

 two— that is to say, the direct or immediate, and the indirect 

 effects — will also remain constant ; but if, under the same 

 circumstances, one of these factors, say the direct effect, 

 should for any reason be enhanced, we might then expect 

 that its complement, the indirect effect, would undergo a 

 corresponding diminution ; while, if the direct effect should 

 be small, the indirect effect would show augmentation. 



These theoretical considerations are found very strikingly 

 verified in the experiment which I shall now describe. I 

 first took a balanced record of growth in a specimen of 

 Crinum Lily, at 30 C. This was then subjected to thermal 

 shocks for five seconds. The direct response, as will be seen 

 from the first record in fig. 185, which is reduced to one-third 

 of the original, was a retardation of growth represented by 

 33 divisions. On the cessation of stimulus, however, the rate 

 of growth did not at once return to the normal, but exhibited 

 the effect of absorbed energy by an acceleration shown in the 

 down curve, and represented by 13 divisions, after which it 

 became normal. The same stimulus was now repeated, and 



