Note 172 — Chapter II

As to the influence of heat and moisture on the geographical distribution of plants, see Henslow's Botany, pp. 295–300, and Balfour's Botany, pp. 560–563. Meyen (Geog. of Plants, p. 263) says, ‘I, therefore, after allowing for local circumstances, bring the vegetation of islands also under the law of nature, according to which the number of species constantly increases with increasing heat and corresponding humidity.’ On the effect of temperature alone, compare a note in Erman's Siberia, vol. i. pp. 64, 65, with Reports on Botany by the Ray Society, pp. 339, 340. In the latter work, it is supposed that heat is the most important of all single agents; and though this is probably true, still the influence of humidity is immense. I may mention as an instance of this, that it has been recently ascertained that the oxygen used by seeds during germination, is not always taken from the air, but is obtained by decomposing water. See the curious experiments of Edwards and Colin in Lindley's Botany, vol. ii. pp. 261, 262, London, 1848; and on the direct nourishment which water supplies to vegetables, see Burdache's great work, Traité de Physiologie, vol. ix. pp. 254, 398.