478 GENERATION OF HEAT. 



each of these various cases, the quantity of water arising from its melt- 

 ing should be the same at the close of the specified number of days. In 

 this case the true element is introduced the element of quantity, as de-' 

 termined by one of the ordinary calorimetric methods. 



It is not, however, to be inferred from this criticism that the peculiar 

 quality of heat which we recognize indifferently by the terms intensity, 

 temperature, or degree, is without significance in the case of plants : the 

 limiting maxima and minima between which a given plant can exist 

 prove that both conditions exert an influence, though they exert it in a 

 different way. Doubtless a plant, from the time of its germination to 

 that of the completion of its organic life, must have a definite quantity 

 of heat measured out to it, but its organic functions might be fatally in- 

 terfered with if the temperature should rise above a limiting maximum, 

 or sink beneath a minimum. 



The definite quantity of heat in this manner demanded by each plant 

 is probably connected with a purely mechanical effect the necessity for 

 the evaporation of a definite quantity of water by the leaves. The inor- 

 ganic salt substances required by every plant are introduced through its 

 roots in a state of solution in water, and, since these salts are mostly of 

 sparing solubility, a great quantity of water is required to accomplish 

 the object. Nevertheless, they are dissolved at a given heat-degree in 

 an invariable proportion in the liquid, and are required by the plant in a 

 determinate proportion as compared with its mass ; so that, were there 

 no other reason, this doubtless would be sufficient to account for the cir- x 

 cumstance under consideration. 



It should also be remembered that every plant generates a certain 

 Disturbance amount of heat, which varies with its organic condition at the 

 arising from time. The experiments of Professor Paine present this in an 

 tion ofheat interesting point of view. The following extract is from the 

 in plants. Medical and Physiological Commentaries, vol. ii., p. 75 : 



"On the 9th of April, 1839, we repaired to a forest in New Jersey, 

 Prof. Paine's provided with very delicate thermometers, of Fahrenheit's 

 experiments. sca l e) constructed for our object. The bulbs were no larger 

 than the stem, the range of the mercury extensive, and the degrees 

 marked upon the glass. The stems filled exactly the bore of a small 

 spiral auger, and when the glass was introduced the air was excluded by 

 applying a silk handkerchief around the hole. The perforations were all 

 made on the northern side of the trees. Fifteen minutes, at least, were 

 allowed for the subsidence of the heat that arose from the friction of the 

 perforator, and the thermometer was generally reapplied at different in- 

 tervals afterward. The perforations were made about four feet above the 

 ground, and the diameters of the trees were ascertained at this point. 

 When the diameter was five inches or more, the perforations were made 



