DEVELOPMENT OF HEAT BY PLANTS. 857 
both of which processes are accompanied by a diminution of 
heat, it is evident that such changes must have some effect in 
modifying the temperature ; and hence if, at such periods, their 
temperature be above that of the surrounding air, that it is due 
to external influences, such as the sun’s rays, and the tempera- 
ture of the soil, &c. This probably explains, to some extent at 
least, why the temperature of thick trees exposed to great heat 
is lower than that of the surrounding air, for at such a period 
vegetation is in a very active condition, evaporation and assimi- 
laticn being then in full play. Again, when the temperature of 
the air is low, as in winter or during the night, little or no 
evaporation or assimilation takes place, and hence we find that 
the temperature is then higher than that of the external air. 
The conclusions in the last paragraph do not, however, alto- 
gether agree with the published result of experiments made by 
Dutrochet ; for he found, by operating with Becquerel's thermo- 
electric needle, that when plants were placed ina moist atmo- 
sphere so as to restrain evaporation, a slight increase of tempera- 
ture took place, thus seeming to prove that the chemical changes 
taking place in plants produced a rise rather than a diminution 
of temperature. Probably this slight increase of heat under 
such circumstances is due to the oxidation or combustion of a 
portion of the carbon of the plant. But Dutrochet found 
that when evaporation was allowed, the proper vital or specific 
heat of plants was slightly below that of the atmosphere. He 
also noticed that the heat of plants varied during the course 
of twenty-four hours,—the hour of maximum temperature 
varying from ten in the morning to three in the afternoon, the 
minimum occurring at midnight. The variation in such cases 
was, however, extremely small, being only from about one-tenth 
to a little over one-half a degree of Fahrenheit. This specific 
heat of plants could only be observed in green and soft struc- 
tures, those which were hard or woody (i.e. those which were 
composed mainly of dead tissues) not possessing any specific 
heat, because in such parts little or no metabolic changes were 
going on. 
The above is but a brief summary of the conclusions which 
have been at present arrived at with regard to the develop- 
ment of heat by plants, and these are by no means of a con- 
clusive nature. Much further investigation is required upon 
this matter. 
In connexion with the subject of heat developed by plants 
may be mentioned the researches of Boussingault, Alphonse 
de Candolle, &c., as to the temperatures required by ditterent 
plants to stimulate them into vegetative or reproductive activity. 
That a certain sum of heat is required for the proper develop- 
ment of a plant has long been known ; also that the life-history 
of some plants (as Wheat) will be completed in a shorter time 
in hot than in more temperate climates. 
