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
