1 84 PLANT RESPONSE 



in each of these cases, at the standard rate of i° per 15 

 minute, that not only is there a definite death-point, 

 evidenced by sudden contraction, but that the discoloration- 

 point is separated from this, by a definite temperature-interval. 

 And since we have found the death-point to be translocated 

 by the influence of various agencies, we may expect the 

 discoloration-point also to be displaced, in a similar manner, 

 under the same conditions. 



Development of thermographs. — This being so, it ought 

 to be possible to ' develop ' images of local death. We take 

 a coloured petal, say of Passiflora, and placing two circular 

 electrodes diametrically opposite to each other, with the petal 

 between, pass tetanising shocks, which are of sufficient 

 intensity to fatigue, but not to kill, the tissue. When the 

 electrodes are removed, there is nothing by which the eye 

 may distinguish the zone of fatigue. In order now to 

 develop this invisible picture, we have simply to subject the 

 specimen to the ordinary bath, with rising temperature. For 

 we have seen, from experiments already described, that the 

 power of the tissue to resist death is lowered by fatigue. In 

 the case of the present specimen, therefore, the fatigued area 

 will die, and undergo subsequent discoloration, earlier than 

 the rest of the petal. In carrying out this experiment, the 

 area of fatigue was found developed as a white image on a 

 purple background, at a temperature of 45° C. It should be 

 remembered that, as said before, the lowering of the death- 

 point varies with the amount of the fatigue ; hence the point 

 of discoloration may be found as low as 40° C. or as high as 

 50° C. and upwards. If the petal be removed from the bath 

 as soon as development begins, the image will remain. But 

 if it be maintained under the rising temperature, the thermo- 

 graph will vanish, with the death and discoloration of the 

 background. 



Again, I took two similar styles of Datura. One of these 

 I kept as standard, and passed tetanising shocks through the 

 other. On subjecting both to rising temperature in a single 



