368 THE PRODUCTION OF HEAT, LIGHT, AND ELECTRICITY 



Plants are able to grow when their temperature is, owing to transpira- 

 tion, kept permanently below that of the surrounding medium, and their 

 growth is not appreciably affected by their own production of heat. The 

 latter is therefore merely an accessory result of metabolism, and has no 

 special economic value, but merely indicates a more or less pronounced 

 preponderance of exothermic chemical changes. The heat-vibrations pro- 

 duced in this way in the interior of the protoplast may, however, have quite 

 a different value and importance to the heat derived from without. 



In certain cases a rise of temperature may be of definite advantage to 

 the plant. For instance, the warmth of the spadix of an Aroid may aid in 

 the rapid development of the pollen, and also in attracting pollen-carrying 

 insects l . Furthermore, the production of heat enables feeble transpiration 

 to continue in air saturated with moisture, and hence may aid in preventing 

 the injection of the intercellular spaces with sap. No protection against 

 frost is possible, however, since respiration and the production of heat 

 entirely or almost entirely cease as the temperature sinks below zero centi- 

 grade 2 . On the other hand, the increasing production of heat with rising 

 temperature instead of being advantageous may cause the plant to be more 

 rapidly fatally affected. 



Owing to their relatively less surface, fleshy or bulky organs are 

 appreciably warmed by a smaller production of heat than thin membranous 

 ones. The aggregation of different parts as well as the provision of hairy 

 or non-conducting coverings by lessening the loss of heat will cause the 

 temperature to rise. In fact a heap of living respiring plants in saturated 

 air will always show a temperature appreciably higher than that of the 

 surrounding air. Furthermore, the rise of temperature in the interior will 

 excite more active respiration if the aeration is sufficiently rapid. The 

 heating of the heaped grass cut from a lawn, as well as of imperfectly dried 

 hay in hay-ricks, is in the first instance due to the plants' own warmth, 

 although the subsequent more pronounced heating is largely due to the 

 rapid development of micro-organisms at the raised temperature. 



The actual amount of heat produced by a plant can only be determined 

 by calorimetric measurement, but such estimations give no idea as to the 

 exothermic and endothermic chemical changes which may go in the plant 3 . 

 Even when such substances as starch or sugar form the main material con- 



1 Cf. Ludwig, Biologic, 1896, p. 261 ; G. Kraus, Die Bluthenwarme bei Arum italicunt, 1882, 

 p. 20 (reprint from Abhandlg. d. naturf. Ges. zu Halle, Bd. 16) ; Ann. d. Jard. hot. de Buitenzorg, 

 1896, T. xni, p. 271). 



3 Seignette (Revue generate de Bot., 1889, T. I, p. 614) observed in the case of bulbs and tubers 

 a greater difference of temperature at 6C. than at 3C. and nC., but this was probably due to 

 special causes. Cf. H. Dixon, Transact, of the Irish Academy, 1903, T. xxxil, Part III, p. 145. 



3 Cf. Pfeffer, Studien zur Energetik, 1892, p. 189 ; Ostwald, Lehrb. d. allgem. Chemie, 2. Aufl., 

 1893, Bd. n, p. i. 



