152 PLANT RESPONSE 



of the tissue was somewhat depressed, this spasmodic move- 

 ment was found to take place at 53 C, which exactly agrees 

 with that of radish, under similar conditions. That this was 

 the true death-point of the Mimosa specimen was proved 

 when, on trying the electrical test, it was found that the 

 normal electrical response had disappeared. If, again, one 

 branch of Mimosa on the intact plant be bent over, and 

 subjected in the manner described to the death-temperature, 

 we find, on examination after a considerable lapse of time, 

 that whereas the leaves of the rest of the plant are still fresh 

 and healthy, reacting to stimulus, those of this branch may 

 be seen, from their dried and shrivelled condition, to be 

 quite dead. 



Death-response a true physiological response. — It will 

 be shown that this death-response is a true physiological 

 response. Under normal conditions, it will be found to be 

 extremely definite, even in different plants. But, under 

 physiological modification, it varies appropriately with the 

 season, age, condition of freshness or fatigue, and the action 

 of chemical reagents. I shall first, then, demonstrate the 

 effect of age on the death-point. Thus, on immersing a 

 branch of Mimosa in water whose temperature is raised con- 

 tinuously, we find that the spasmodic movement of death 

 occurs earlier in the young leaves than in the old. Young 

 seedlings, again, have a lower death-point than mature plants. 

 The following table gives results bearing on this fact. The 

 death-points of different plants in the same season and of the 

 same age are, however, so definite as to be almost like a 

 physical constant. This will appear from the following 

 tabular results, and also from results given in the next 

 chapter. A fact which will be explained later must be 

 stated here. In these experiments with continuously rising 

 temperature, it is found that the first spasmodic movement 

 downwards is succeeded by a later, upwards, by which time 

 the temperature has risen a few degrees. The temperatures 

 of both movements are given in the accompanying table, 

 corresponding to their occurrence in two leaves, one old and 



