666 GENERAL CONDITIONS OF PLANT-LIFE. 



sperms does require it; and in both cases the change does not take place at low 

 temperatures (see p. 651). 



It may be concluded from such observations as have been made that all the 

 visible parts of the solar spectrum have the power of turning the etiolated grains of 

 chlorophyll 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 ^ 



(d) The Decomposition 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 under favourable circumstances (see p. 651) 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, espe- 

 cially transverse cuts of the stem ; and it has been shown by Pfefifer 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. 664). This evolution is always takings 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 in\-estLgated by Draper and very recently again by 

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

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

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

 number of bubbles. Pfeffer showed fij-st of all * 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. 



^ Draper, Annales de chimie et de physique 1844, p. 214 et seq. — Pfefifer, 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 Midler, Botanische 

 Untersuchungen, Heft I, Heidelberg 187 1, is also discussed. 



