THE VEGETABLE CELL. JOl 



tion of a Large amount of aqueous vapour, as by the retention of an 

 organic substance which should have been excreted by the leaves. 



f. E'volution of heat 



With the nutritive processes of plants is connected their power 

 of producing heat. That plants possess this power may be de- 

 monstrated by simple observations, but these require great ac- 

 curacy and certain rules of precaution, to avoid arriving at false 

 conclusions ; for in determining the proper heat of plants, not 

 only does the mostly very small amount of heat which is capable 

 of raising the temperature of the plant a little above that of the 

 air, render great caution necessary in making the experiments, 

 but, under common circumstances, so much heat becomes latent, 

 through the active excretion of aqueous vapours from the leaves, 

 that the temperature of the plant, in spite of the latter producing 

 heat, sinks below the temperature of the surrounding air. Theie- 

 fore to arrive at accurate results, it is not merely necessary to use 

 a very sensitive thermometric apparatus, but also to cut off the 

 refrigeration by evaporation. 



That seeds when germinating, as they lie heaped in large masses, 

 evolve a considerable degree of heat, is a fact long known from 

 the malting of grain, but the cause of it was incorrectly sought 

 for in a process of fermentation. To Goppert f'" Ueher Warmeen- 

 hvickelung in der lehenden Pflanze") is due the merit of having 

 demonstrated that such is not the ease, but that the evolution of 

 heat is connected with the process of germination. Seeds of very 

 different chemical composition (of different grains, of Hemp, 

 Clover, Spergula, Bmssica, &c) made to germinate in quantities 

 of about a pound, became heated, at a temperature of the air of 

 ^S°— 66°, to 59—120° Fahr. 



It was likewise shewn by Goppert, that full-grown plants, also, 

 such as Oats, Maize, Gyperus esculentus, Hyoscyamus, Sedum 

 acre, &c., laid together in hcaps^ and covered with bad conductors 

 of heat, cause a thermometer placed among them to lise about 

 2° — 7^" (Spergula ate much as 22°) above the temperature of the 

 air. Dutrochet succeeded, with the help of BecquereFs thermo- 

 electiic needle, in demonstrating an evolution of heat in plants 

 standing alone (" Ann. d. sc, not" 1 8S9, ii. 77) ; but here the cold of 

 evaporation must be cut off by placing the plant in an atmosphere 

 completely saturated with water. Under these circumstances, the 

 temperature of all vegetating parts, the roots, the leaves, the 

 young juicy shoots (but not those of hard wood), were elevated 

 from about one l-6th to 1-1 2th of a degree. The evolution of 

 heat exhibited a daily maximum and minimum ; the latter oc- 

 curred about midnight, the former about noon, yet not at the 

 same hour in different plants, for the time varied from 10 AM.. 

 to 2 P.M. 



