INFLUENCE OF THE DIFFERENT RAYS OF THE SPECTRUM 345 



approximately accurate primary curve of assimilation to be constructed, and 

 it is easy to understand why precisely similar results may not always be 

 obtained. 



The composition of the light alters as it penetrates each successive 

 chlorophyll-layer, and the assimilatory curve and total assimilatory effect 

 change in a corresponding degree, the maximal point of the secondary 

 assimilatory curve being displaced towards D. Hence various authors 

 (Draper, Pfeffer) found that under strong illumination the maximal assimi- 

 lation occurred in yellow light, so that the assimilatory curve appeared to 

 run parallel with the visual intensity of the light 1 . It is easy to show that 

 the course of assimilation and absorption is such that the total result pro- 

 duced by light of varying wave-length is different under different condi- 

 tions, and that as the intensity of the light changes so also does the 

 assimilatory curve alter. The variations which the secondary assimilatory 

 curve undergoes under varying external conditions can therefore only 

 be determined by direct experiment in each case. 



The yellow rays, which penetrate more deeply, may produce a greater 

 total assimilatory effect than the rays between B and C, which are more active, 

 but which are also more rapidly absorbed and thus act upon fewer chloro- 

 plastids than the yellow rays do ; while if the red rays cause less carbon 

 dioxide to be decomposed than the yellow rays in relation to the amount 

 of light absorbed, as appears to be the case (cf. Ass. and Abs. Green in 

 Fig. 51), then the value of the latter will be still further enhanced when 

 light penetrates a chlorophyllous tissue 2 . It is of the utmost importance 

 that the light which has already passed through chloroplastids should still 

 be able to be used in assimilation, for it is impossible in an ordinary 

 leaf for all the chloroplastids to be exposed to direct illumination. 



Both the absorption and reflection of light by chlorophyll are of importance, 

 and the curves given in Fig. 51 give a general idea of the former. The spectra 

 of a green leaf and of a chlorophyll extract correspond, except for slight displace- 

 ments due to the influence of the solvent media, details of which, as well as of the 

 yellow pigments of the chloroplastids, are given in the literature quoted *. Even 



1 Cf. Pfeffer, Bot. Zeitung, 1872, p. 425 ; Arb. d. Bot. Inst. in Wiirzburg, 1871, Bd. I, p. r, 

 and Physiol., i. Aufl., Bd. I, p. 214. Reinke (Bot. Zeitung, 1884, p. 39) obtained with Elodea, 

 by the bubble-counting method, a curve corresponding more closely with the Ass. Green in Fig. 51, 

 except that no secondary maximum was observed in the blue, for which see Kohl, Ber. d. D. Bot. 

 Ges., I.e. In these and the other researches quoted, no distinction has been made between the 

 primary and secondary assimilatory curves. The curve Ass. Red in Fig. 51 corresponds fairly well 

 to the optical intensity of the light. 



[Pennington (Contrib. Bot. Lab. Univ. Pennsylvania, I, 1897, p. 203) states that Spirogyra 

 forms no starch in blue or yellow light, but this was probably owing to the employment of light of 



feeble intensity.] 



s Vierordt, Die Anwendung d. Spectralapparates zur Photometric, 1873 ; N. J. C. M ler, Bot. 

 Unters., 1876, p. 325; Wolkoff, Die Lichtabsorption in Chlorophylllosungen, 1876; Engelmann, 



