Page:History of botany (Sachs; Garnsey).djvu/528

508 whole mass of facts accumulated by Goeppert in his book of 1830, from which he tried to prove (p. 228) that plants at no period of their life possess the power of generating heat a view which he retracted however in 1832, when he had observed a rise of temperature in germinating plants, bulbs, tubers, and in green plants, when collected into heaps. How difficult it was for physiologists under the dominion of the ' vital force ' to hold firmly to the simple principle of natural heat, and not to be led away by isolated observations, is shown by the expressions of De Candolle in 1835, and still more by those of Treviranus in 1838. It is therefore refreshing to see Meyen in his 'Neues System' (1838), vol. ii, warmly asserting this principle, and making the development of heat in plants a necessary con- sequence of their respiration and of other chemical processes. Meyen himself produced no new observations; but Vrolik and De Vriese showed by laborious experiments in 1836 and 1839 the dependence of the generation of heat in the flowers of Aroideae on the absorption of oxygen. A higher importance as regards the general principle attaches to the attempt of Dutrochet in 1840 to prove that even growing shoots generate small quantities of heat, as shown by a thermo-electric apparatus. Some of the details in these observations are open to objection; but it cannot be denied that they are based on a clear recognition of the general principle, though they ignore the consideration that the generation of heat in plants is not necessarily accompanied with a rise in temperature, since cooling causes may be acting at the same time with greater effect. However the doctrine of the natural heat of plants was in the main established by the observations of de Saussure, Vrolik, De Vriese, and Dutrochet, and by Meyen's and Dutrochet's assertion of the principle laid down by Lavoisier, though thirty years elapsed before it became an accepted truth in vegetable physiology.

The crude idea of a vital force was deprived of one of its chief supports when it was recognised that the natural heat of