In the history of science, vegetable life, along with animal life, was one of the two divisions of living matter used in the pre-Darwinian days of chemistry and biology. With the development of the new science of "thermo-dynamics" in the 1850s, those such as William Thomson, began to postulate on how the new mechanical theory of heat and dissipation related to to "vegetable life" or to the "will of animate creatures". [1]

Vegetable life and caloric
In 1787, French chemist Antoine Lavoisier stated that the true elements of vegetables are hydrogen, oxygen, and charcoal and that, in relation to the decomposition of vegetable (and animal) substances by the action of fire that "of these elements, hydrogen and oxygen have a strong tendency to unite with caloric, and be converted into gas, whilst charcoal is a fixed element, having by little affinity with caloric." [2] The theory of the "caloric", as was developed by Lavoiser, was used in 1824 by French physicist Sadi Carnot in his Reflections on the Motive Power of Fire, the founding paper of thermodynamics, in which he supposed that no change occurs in the working body during one engine cycle. [3] This latter assumption, prompted German physicist Rudolf Clausius to spend 15-years developing of the concept of entropy to show that change does occur in the working body. [4]

References
1. (a) Quote: according to “known facts with reference to the mechanics of animal and vegetable bodies” there is “at present in the material world a universal tendency to the dissipation of mechanical energy” and that “any restoration of mechanical energy, without more than an equivalent of dissipation, is impossible in inanimate material processes, and is probably never effected by means of organized matter, either endowed with vegetable life or subject to the will of an animated creature”
(b) Thomson, William (Lord Kelvin), "On a Universal Tendency in Nature to the Dissipation of Mechanical Energy" (Google Books) (URL), Proceedings of the Royal Society of Edinburgh for April 19, 1852, also Philosophical Magazine, Oct. 1852, also Mathematical and Physical Papers, vol. i, art. 59, pp. 511.
2. Lavoisier, Antoine. (1787). Elements of Chemistry - in a New Systematic Order, Containing all the Modern Discoveries, (ch XII, pg. 123). New York: Dover (reprint).
3. Carnot, Sadi. (1824). “Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power.” Paris: Chez Bachelier, Libraire, Quai Des Augustins, No. 55.
4. Clausius, R. (1865). The Mechanical Theory of Heat – with its Applications to the Steam Engine and to Physical Properties of Bodies. London: John van Voorst, 1 Paternoster Row. MDCCCLXVII.