1343. The influence of the different solar rays on vegetation is not yet thoroughly understood; but it is quite evident that the heating and the illuminating rays produce different effects. 'Chemical action,' observes Professor Solly, 'is caused in the cells of the leaves by the agency of these chemical rays of light, which enable the vital power, as it is called, to form organic matter; hence it is frequently said that light stimulates vegetation. Chemical action is also augmented by heat, which, though it cannot alone effect those changes usually produced by light, considerably assists them, and is, therefore, classed with light as a stimulus.' (Rural Chemistry, 2nd ed., p. 153.)
