744 GENERAL CONDITIONS OF PLANT-LIFE. 



granules of Angiosperms green; but that the yellow rays and those nearest to 

 them on each side are the most powerful ; and that this is also the case with the 

 exhalation of oxygen from cells containing chlorophyll \ 



(<$) T/ie Decompositio7i of carbon dioxide in cells containing chlorophyll, on 

 which depends the assimilation of plants, and which is perceptible externally by 

 the exhalation of a volume of oxygen nearly equal to that of the carbon dioxide 

 absorbed, is brought about at a favourable temperature (see p. 729) by rays of light. 

 In submerged water-plants the gas (always mixed with a larger or smaller quantity 

 of nitrogen) escapes in the form of bubbles from wounds, especially transverse cuts 

 of the stem; and it has been shown by Pfeffer and myself that when their size 

 is constant the rapidity of these bubbles, i.e. the number of them formed in a unit of 

 time, may even be used to give an exact measurement. In observations on land- 

 plants it is on the other hand necessary to expose the leaves to light together with 

 air containing carbon dioxide in glass vessels of a suitable size and form, and to 

 measure the quantity of gas by a eudiometer. 



The smallest intensity of light necessary for the evolution of oxygen is — 

 judged by the subjective measure of its brightness to our eye — rather considerable 

 (see p. 742). This evolution is always taking place with considerable energy in 

 diffused daylight, even when the rays reach the plant only from a small portion of 

 the sky ; but it is much stronger in direct sunlight. 



The specific effect on the evolution of oxygen of the variously refrangible 

 elements of sunlight, in other words of the different coloured bands of the solar 

 spectrum, has been carefully investigated by Draper and very recently again by 

 Pfeffer ^. The observations were made partly with the solar spectrum, partly with 

 solutions of different colours which transmitted light of a particular refrangibility. 

 The amount of gas exhaled was measured partly by the eudiometer, partly by the 

 number of bubbles. Pfeffer points out ' that each portion of the spectrum exercises 

 a specific quantitative influence on the power of assimilation ; and that this remains 

 unchanged whether the particular rays act separately on the parts of plants that 

 contain chlorophyll, or combined with some or with all the other rays of the 

 spectrum.' 



The following additional result was also obtained from Draper's and Pfeffer's 

 observations, and from mine already quoted : — ' Only those rays of the spectrum 

 which are visible to our eye have the power of decomposing carbon dioxide; and 

 indeed those which appear brightest to the eye, the yellow rays, are alone as 



^ See in particular Guillemin, Ann. des Sci. Nat. 1857, vol. VII. p. 160. [According to Wiesner 

 (Unters. ueb. d. Beziehungen des Lichtes zum Chlorophyll, Sitzber. d. Wien. Alcad., vol. 69, 

 1874; also Bot. Zeitg. 1874), etiolated plants become green much more rapidly in blue than in 

 yellow (intense) light. He attributes this to the more active decomposition of the chlorophyll in the 

 yellow light. This view is supported by the observation of Guillemin (Ann. d, Sci. Nat., 1854) 

 and of Famintzin (Melanges biologiques, Acad. Imp. de St. Petersbourg, vol. 6, 1866) that the 

 leaves of etiolated plants become green more rapidly in diffuse daylight than in sunshine.] 



'^ Draper, Annales de chimie et de physique, 1844, p. 2\\et seq. — Pfeffer, Arbeiten des Botan- 

 ischen Instituts in Wiirzburg, Heft I. p. 48, where reference is also made to the whole of the rest 

 of the literature.'— Pfeffer, Sitzungsber. der Gesellsch. zur Beforderung der gesammt. Naturwiss. zu 

 Marburg, 1872, May 16; and Bot. Zeitg. 1872, no. 23 et seq., where the paper by Miiller, Botanische 

 Untersuchungen, Heft I, Heidelberg 1 871, is also discussed. [For an account of Draper's researches 

 into the relations existing between plants and light, see his Scientific Memoirs, London, 1878.] 



