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 ^ 
ib) The Decomposiiion 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 Sei, Nat. 1857, vol. VII, p. 160. [According to Wiesner 
(Unters, ueb, d. Beziehungen des Lichtes zum Chlorophyll, Sitzber. d. Wien, Akad,, 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. Sei, 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, Annates de chimie et de physique, 1844, p. 214 ssq. — Pfeffer, Arbeiten des Botan- 
ischen Instituts in Würzburg, Heft I. p. 4S, where reference is also made to the whole of the rest 
of the literature, — Pfeffer, Sitzungsber. der Gesellsch, zur Beförderung der gesammt, Naturwiss. zu 
Marburg, 1872, May 16; and Bot. Zeitg. 1872, no, 23 et seq., where the paper by Müller, 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.] 
I 
