DEVELOPMENT OF HEAT. 541 



rise of the sap in the plant, and from the discoveries of De la Rive and 

 Alph. DeCandolle *, from which it appears that wood in its longitudinal 

 direction is a good, but across its fibres a very bad, conductor of heat. 

 It is especially necessary that a greater number of comparative observa- 

 tions should be made ; first, in plants the roots of which attain different 

 depths ; then in herbaceous and woody plants ; and, finally, in tropical 

 plants, which latter we shall probably be only able to obtain when 

 governments begin to send out naturalists instead of collectors for their 

 museums. A physiologist properly supported, and making a good use of 

 his time, would do more for science by a residence of two years in the 

 forests of the Orinoco than all the travels that have been undertaken since 

 the time of A. von Humboldt. 



Observations on the rise of temperature during flowering have hitherto 

 been instituted on the Aracecz f alone. Lamark observed this fact in 

 1777 in Arum Italicum. Sennebier, Bory St. Vincent, and others^: 

 subsequently communicated observations on the subject. The most exact 

 and elaborate investigations are those of Vrolik and De Vriese. Ac- 

 cording to them, the temperature has a regular periodicity within the 

 twenty-four hours, and attains its maximum in the afternoon, between the 

 hours of two and five. The difference between the temperature of the 

 atmosphere and that of the root is sometimes as much as from 20 30 R. 

 In this case, also, the probability is that the temperature is the result of a 

 process of combustion. According to Th. de Saussure, the root of an 

 Arum maculatum changed thirty times its volume of oxygen into carbonic 

 acid in twenty-four hours. We are deficient, however, in comprehensive 

 comparative observations, which should be made on crowded flower- 

 stalks. The chemical processes ought to be measured with the greatest 

 accuracy, and the temperature developed ought to be calculated and com- 

 pared with the temperature observed. In all the cases which we have 

 enumerated, the absolute temperature depends on the intensity of the vital 

 process, and is higher in proportion to the vigour of the vegetation of the 

 plants, or in proportion to the absorption of the sap and the vigour of its 

 chemical processes. 



Of these three phenomena, the first and last seem to have the same 

 origin ; the second is independent. Meyen maintains that the production 

 of temperature in plants is peculiar, which may perhaps be due to the 

 chemical processes that are constantly going on. But no result can be 

 gained in the rude manner in which he pursues the subject. It is 

 merely a guess to say that the temperature in the interior of trees must 

 depend on the same causes as the development of temperature during 

 germination and flowering. Thus much is certain, that, during the pro- 

 cesses of germinating and flowering, carbonaceous matters are consumed 

 and carbon is burned. In the process in the stem it is also certain that 

 a formation of purely carbonaceous substances takes place, and it is as yet 

 quite uncertain whether the chemical processes present absorb or liberate 

 heat, because we are not yet acquainted with those processes. Meyen 

 doubts the rising of the sap in winter, because roots are frequently found 

 thoroughly frozen. But what roots ? The difference of temperature 

 between day and night disappears at a depth of 3', and that between 



* Poggendorff's Annalen, vol. xiv. p. 590. 



f A complete enumeration of all these observations are to be found in the " Flora" 

 (1842, vol. i. Supplement, No. 6, p. 84.). 

 \ Meyen, Physiologie, vol. ii. p. 184. 

 Annales de Chimie et de Physique, vol. xxi. p. 279. 



