312 LECTURE XIV. 



temperature is moderately high, so that, within a certain 

 limit, the excess of the temperature of the plant over that of 

 the air is greater at a higher than at a lower external 

 temperature. We can readily understand this, for we know- 

 that destructive metabolism is more active, within a certain 

 limit, the higher the external temperature. When the varia- 

 tions of the external temperature are gradual, the course of 

 the daily period asserts itself even in opposition to them : 

 but when they are sudden and considerable, they give rise, as 

 Hoppe found in the case of Colocasia odora, to corresponding 

 variations, but of greater amplitude, in the daily period of the 

 plant. With regard to the influence of light, Dutrochet 

 found that the daily period persists for a time (4 days, 

 Campanula medium ; 3 days, Lactuca sativa ; I day, Borago 

 officinalis) when the plant is kept continuously in darkness, 

 and then disappears, and that it is slowly restored when the 

 plant is again exposed to light. There can be little doubt 

 that this influence of light is an indirect one ; that the 

 disappearance of the daily period is due to a diminished 

 destructive metabolic activity in consequence of the arrest of 

 constructive metabolism. We met with a case of this kind in 

 a previous lecture (p. 261). 



The observations which we have hitherto selected in illus- 

 tration of the evolution of heat are all such as have been 

 made on growing organs, for it might be expected, and 

 investigation has fulfilled the expectation, that, inasmuch as 

 growth is accompanied by active destructive metabolism, it 

 would be attended by an evolution of heat. This appears 

 to be true also with regard to movement, for Bert has 

 observed that the movement of the motile organ of the 

 leaf of Mimosa pudica is accompanied by a rise of tempera- 

 ture. But it must not be supposed that the evolution of 

 heat in plants is confined to such periods of specially active 

 destructive metabolism. Heat is evolved whenever destruc- 

 tive metabolism is in progress, and the only reason why we 

 cannot in all cases detect an evolution of heat is that the 

 destructive metabolism is not sufficiently active to determine 

 such an evolution of heat as to make good the loss of heat 



