THE PRODUCTION OF HEAT BY ANAEROBIC METABOLISM 379 



Hence the lactic and butyric fermentations yield heat, although the observed amount 

 is less than that theoretically calculated unless the heats of solution are taken into 

 account. In addition the by-products of fermentation will influence the liberation 

 of heat according to their character and properties. Most metabolic processes are 

 attended by a trifling production of heat, and in fact it is even possible that metabolism 

 may in some cases be attended by an absorption of heat l . 



SECTION 83. The Temperature of the Plant under Normal Conditions. 



External and internal radiation, the conduction and production of 

 heat, the temperature of the surrounding medium, and the activity of 

 transpiration are among the factors regulating the temperature of the 

 plant, and they do not affect the different organs of the plant alike. 

 Hence the temperature of a root or of a shaded organ is usually different 

 to that of the stem or of an insolated organ. In such cases a slow 

 transference of heat may occur from the hotter to the colder organ by 

 conduction, or by convection or transpiration currents of water. 



Small or slender organs rapidly assume the temperature of the surround- 

 ing medium, but hours may elapse before the full effect of a change of 

 temperature in the external medium is shown at the centre of a tree- 

 trunk or of a large tuber. Neighbouring regions may indeed be at widely 

 different temperatures, if one part is insolated but the other not, or if 

 one part projects above water but the other is submerged. Plants are 

 able to grow in spite of these local and general variations of temperature, 

 if they are not too pronounced. 



It must be remembered that under constant external conditions the 

 activity of transpiration may undergo autogenic modification, and that 

 its cooling effect will alter correspondingly. Gaseous exchanges may be 

 modified in the same way, but these have a much feebler influence upon the 

 body-temperature. Fleshy objects often become hotter in sunlight than the 

 exposed bulb of a thermometer. Thus Askenasy 2 observed a temperature 

 of 52 C. when the thermometer-bulb was inserted between the resetted 

 leaves of Semper vivum alpinum (shade temperature = 28-iC.), whereas 

 a thermometer pressed against the thinner leaves of Gentiana cruciata 

 or between the tufted leaves of Aubrietia dcltoidea showed a temperature 



1 Cf. Pfeffer, Studien zur Energetik, 1892, p. 189. Nageli incorrectly supposed that all enzyme 

 action was accompanied by an absorption of heat, and proposed to make this a distinction from 

 ' vital ' fermentation accompanied by a production of heat. Cf. Hertzog, 1. c. 



8 Askenasy, Bot. Ztg., 1875, p. 441. Cf. also Haberlandt, Sitzungsb. d. Wiener Akad., 1892, 

 Bd. ci, Abth. i, p. 787; Passerini, Nuovo giornale bot. italiano, 1901, vol. vni, p. 69. Rameaux 

 (Ann. sci. nat., 1843, 2* s^r., T. XIX, p. 21) observed 33 C. registered by a thermometer whose bulb 

 was inserted in a thin insolated branch, whereas with the bulb in sunlight 24 C. was shown. 

 Becquerel (Compt. rend., 1858, T. XLVII, p. 717) observed 37 C. at the centre of a thick stem 

 exposed to sunlight. For the older literature see Goppert, Die Warmeentwickelung i. d. Pflanze, 

 1830. 



