FORMS OF ENERGY IN THE PLANT 399 



may easily be obtained ; on the other hand, if these structures be dead, and if 

 micro-organisms be excluded, no such rise of temperature is observable even 

 though all the other conditions be maintained. If it be desired to measure 

 minute differences of temperature in individual organs the best method to 

 employ is the thermo-electric pile, viz. copper and iron wires in the form of 

 needles, soldered together, and varnished over, the one stuck into the part to be 

 investigated, the other being in the air or in another organ for comparison ; the 

 apparatus is connected with a galvanometer, from the movements of whose 

 indicator the difference in temperature between the needles may be readily 

 determined (Dutrochet, 1840). 



This method is merely qualitative and shows whether heat is developed or not ; 

 the amount of heat produced can be estimated only by minute calorimetric 

 investigations, the carrying out of which presents many difficulties. According to 

 G. Bonnier (1893) ikg.of germinatingseeds or young seedlings can evolve twenty, 

 fifty, or even a hundred calories or more per minute. This is a quite considerable 

 amount, for one calorie is the amount of heat necessary to raise i g. of water 

 from 0° C. to 1° C. Both calorimetric and thermometric measurements show 

 that not only the conditions of the plant itself but also those of the surroundings 

 play a great part in heat production. We must base our statements on thermo- 

 metric calculations, for as yet there are very few calorimetric measurements 

 forthcoming. 



The production of heat depends to a remarkable degree on the state of 

 development of the organ of the plant under investigation ; generally speaking, 

 growing points and young members produce more heat than the same parts 

 when mature. Still Dutrochet (1840) was able in many plants to establish that 

 an excess of o-i° to 0-3° C. over the temperature of the air existed in the mature 

 stem if transpiration were prevented. A similar phenomenon is observable in 

 mushrooms, whilst in leaves and fruits evolution of heat is generally more re- 

 stricted. There are, however, organs which when mature exhibit a maximum 

 production of heat and the highest temperatures of all have been observed in 

 fully-developed parts of flowers or inflorescences. Indeed, it is often sufficient 

 merely to feel such organs to be convinced that an evolution of heat is taking 

 place. By means of a thermometer it has been shown that the inflorescences of 

 Palmaceae and Cycadaceae and certain parts of the flower of Victoria regia not 

 infrequently possess a temperature 10° or more above that of the air, while in 

 the Araceae much higher temperatures have been obtained. Thus Kraus (1894) 

 found that a thermometer placed in the large spadices of Arum italicum gave 

 a maximum temperature of from 49-2° to 51-3° C., or 33-2° to 35-9° higher than 

 that of the air. In nature transpiration brings about a marked cooling effect, 

 since the plant could not for long have maintained such temperatures as were 

 found. Special adaptations may certainly arise quite generally, for we know 

 of Bacteria which are characterized by their high maximum temperature and 

 such forms produce a considerable amount of heat (Cohn, 1893). 



Amongst external factors temperature itself is entitled to receive special 

 attention, since the production of heat by the plant is not entirely independent 

 of external heat. As in the combustion of carbon, &c., production of heat begins 

 to take place only after a sufficiently high preliminary temperature has been 

 attained. At 5° to 6° C. the buds of Aesculus show no development of heat, 

 but at about 20° C. an excess of 0*63° may be obtained. Germinating wheat, 

 which at 11° C. gave an excess of i-i°, at 15° C. showed a rise of 1-4° C. Syste- 

 matic research on this question is still required, and detailed investigations are 

 especially needed to determine whether a rise in external temperature above a 

 certain point produces once more a diminution of heat production in the plant. 



Not infrequently the evolution of heat exhibits a certain regular periodicity. 

 Thus the young inflorescences of Arum italicum show at first about the same 



