Meyen's Beport for 1 839 on Physiological Botany. 20 



or even y*^° o^ tV°' '^^^ proper heat of young twigs and leaves 

 vanishes during the night, or in general in the dark, and ap- 

 pears again under the influence of light. The higher the ex- 

 ternal temperature, the greater is the vegetable warmth. That 

 part of the heat of plants which is carried oif by the evolution 

 of oxygen cannot be determined quantitatively. 



M. Dutrochet had probably given the above researches into 

 the safe keeping of the Academy, in order to secure to him- 

 self the priority of the discovery ; however, the proofs that 

 plants possess a peculiar heat resulting from their vital acti- 

 vity have been long since published in Germany ; and in the 

 second part of my ' Physiology,' published in the beginning 

 of the year 1838, I have proved that an evolution of heat is 

 exhibited not only by germinating seeds and by the fresh 

 fruits of Areca Catechu when lying together, but also by 

 leaves and herbs in general ; " singly they do not exhibit any 

 warmth on account of the evaporation, but so much the 

 more when they lie together in masses.'' It must not be 

 believed that T state this as a simple idea or theory, for I 

 have convinced myself of its correctness with the thermo- 

 meter in my hand ; I have several times exi)erimented with 

 fresh-cut grass and fresh spinach leaves*. At the same time I 

 showed, that those botanists who wished to prove or disprove 

 the existence of vegetable heat could not choose a worse part 

 for their experiments than the wood. The very low degree of 

 warmth mentioned by M. Dutrochet is not sufficient to prove 

 the existence of vegetable heat ; for in my first Report ( Wieg- 

 mann's Archiv, 1835) I have mentioned some experiments, ac- 

 cording to which bad conductors, as for instance, the wood of 

 chairs and tables in my room, exhibited an excess of heat of 

 2° R. above that of the air, etc. A note which M. Dutro- 

 chet has added to the above letter on the Gth of June 1839, 

 states that his nev/ researches confirm the former ones. In 

 the stem of Euptiorbia lathyris he saw^ the vegetable heat 

 amount to ^° C, but only so long as it was in a verdant state. 

 He also remarked the heat in the roots, fruits, and even in 

 the embryos. Complete exclusion of light totally prevents 

 the rise and fall of temperature, but this does not ah^ ays take 

 place the first day ; M. Dutrochet remarked the change of 

 temperature by night and by day even on the second day of 

 the experiment. 



From some remarks on the above treatise of M. Dutrochctf 

 made by M. Becquerel, it appears that he had used the thermo- 

 electrical apparatus for determining the heat of plants two 



• Is not the accession of licat in this case the result of fermentation 1 — Ed. 

 f Coinptes Rondiis de 17 Jiiiii, p. 'Xl'J. 



