332 M. Van Beck on the Temperature of Plants. 



By presenting distinct and daily periods, I perceive that 

 this heat gradually augments till the afternoon, when it begins 

 to diminish, almost entirely disappearing during the night, 

 and shewing itself again on the following day. 



The maxunum of inherent heat which I observed in this 

 manner, on the 29th September, an hour and a quarter after 

 noon, in a young leaf of Sedum cotyledon, tested by numerous 

 series of comparative trials, did not appear to me much to ex- 

 ceed 0^25 centigrade, while it M^as usually much less. In 

 rainy and dull days, the phenomenon did not display itself so 

 evidently as when the weather was calm and clear. 



Up to this point my experiments perfectly agree with those 

 of M. Dutrochet; but in one respect a difference appeared 

 which it is important to specify. 



M. Dutrochet states himself to have found that, in atmo- 

 spheric air not saturated with humidity, the dead leaf was 

 always found colder than a similar leaf of a living plant ; 

 while my experiments, although repeated in many different 

 forms, have constantly presented to me the greatest cold in 

 the living plant. 



This result did not appear to me singular, for, seeing that 

 the cold which manifests itself on the surface of living plants 

 is certainly produced by evaporation, and that this evapora- 

 tion is at the same time in intimate relation with their vital 

 functions,* it was to be presumed that, notwithstanding the 

 proper heat, which is always very small, of the living plant, 

 the latter would shew itself in a free air, where the evapora- 

 tion might still proceed, at all times colder than the dead leaf, 

 since in the latter evaporation could not take place but by 

 ordinary physical causes, Avhile it is augmented by the vital 

 action in the living plant. 



A singular phenomenon, -which I have always observed in 

 the course of my experiments, is, that in suddenly raising the 



* This relation shewed itself particularly in the experiments which I 

 made some time ago, in connection with Professor Bergsma, on a hyacinth 

 in flower, with its roots plunged in warm water, a report of which we have 

 had the honour to communicate to the Academy. 



