680 GENERAL REVIEW OF THE NUTRITIVE PROCESSES. 



ter of ordinary experience, is probably exerted directly upon the nervous 

 system. 



889. Having thus considered the general facts which indicate the faculty 

 possessed by the living system, in the higher Animals, of keeping up its tem- 

 perature to an elevated standard, and of preventing it from being raised much 

 beyond it by any degree of external heat, we have next to inquire to what 

 this faculty is due. We shall be more likely to arrive at accurate results in 

 such an inquiry, the more comprehensive is our survey of the phenomena to 

 which it relates.* 



a. The most recent experiments on the temperature of Plants (those made by MM. Bec- 

 querel and Breschet with the thermo-multiplier) have demonstrated, that in those parts in 

 which the vital processes are taking place with activity, a sensible amount of caloric is being 

 constantly evolved. The amount of this evolution of heat is generally very low, not more, 

 in fact, than a single degree (Fahr.) ; and as it does not more than counterbalance the effect 

 of the evaporation, which is continually taking place from the surface, there is no sensible 

 difference between the temperature of the plant and that of the surrounding air. At the 

 time of Flowering, however, a much greater degree of heat is generated in many plants, 

 especially in those in which a large number of flowers are crowded together, as in the case 

 of the Arum tribe: thus a thermometer placed in the midst of twelve spadixes has been 

 seen to rise to 121, whilst the temperature of the air was only 06. During the Germina- 

 tion of seeds, again, a considerable development of heat takes place; this, which is soon 

 carried off from a single seed, becomes very sensible when a large number are heaped toge- 

 ther, as in malting; the thermometer plunged into a heap of germinating barley having been 

 seen to rise to 110. 



b. These facts are of more importance than might appear at first sight ; for they indicate 

 unequivocally, that the source of the heat is to be looked for in the Organic functions, not in 

 those of Animal life. The evolution of Caloric has been attributed by many physiologists 

 to the Nervous system ; the influence which this system evidently possesses over the func- 

 tion, being mistaken for the efficient cause of it. As has been remarked on several former 

 occasions, however, the fact that any change takes place in Vegetables, to the same degree 

 (under certain conditions,) with that in which it ever presents itself in Animals, is a suffi- 

 cient proof that it cannot be dependent upon nervous agency, although it may be influenced by 

 it. Moreover, it may be remarked that the production of Heat is an operation of an entirely 

 physical character, and that it may be referred to physical causes; whilst the operations in 

 which the Nervous system is concerned, are such as we cannot liken in any degree to physical 

 phenomena, and are of a purely vital character. In our inquiry into the sources of the Heat 

 evolved by living beings, we are limited, therefore, to those which can operate in the Vege- 

 table kingdom ; and on examining into the phenomena which present any relation to this, 

 we are at once struck with the fact, that an absorption of Oxygen from the air, with an ex- 

 trication of Carlxmic acid, is continually taking place (constituting the true Respiratory pro- 

 cess of Plants, 750); and that these changes occur with excessive activity, at the very 

 periods at which the evolution of Heat is most remarkable, those, namely, of germination 

 and flowering. The quantity of Oxygen consumed by flowers is enormous those of the 

 Arum Italicum having been found to convert 40 times their own bulk of that gas into Carbonic 

 acid, between the periods of their first appearance and their final decay ; and of this, the far 

 larger proportion is consumed by the sexual apparatus, which has been found to consume 132 

 times its own bulk of Oxygen in 24 hours. That this change is a condition necessary for the 

 production of Heat, is fully proved by the fact, that no caloric is evolved when the flowers 

 are excluded from the contact of Oxygen ; whilst the substitution of pure oxygen for atmo- 

 spheric air occasions the elevation of temperature to be more rapid and considerable than 

 usual."|" The same may be said of the heat liberated by seeds in the act of Germination : a 

 large amount of oxygen is absorbed, and of carbonic acid given out, during this process; and 

 the evolution of Heat may be easily shown to be as dependent upon this change, as in the 

 instance just quoted. 



c. When the phenomena of Calorification in Animals are carefully examined, they are 

 found to harmonize completely with this view. Throughout the whole kingdom, an exact 

 conformity may be perceived between the amount of Oxygen consumed and of Carbonic 



acid given off, and the degree of Heat liberated. In the cold-blooded animals, whose tcm- 



* This subject is more fully treated in the Author's Principles of General and Compara- 

 tive Physiology, 548 567. 



"j" See the very interesting experiments of MM. Vrolik and Vriese, in the Ann. des Sci. 

 Nat., N. S. Botan., torn, xi., p. 551. 



