128 



METABOLISM 



been obtained with regard to carbon-dioxide assimilation, however, show that 

 this generahzation is inadmissible, and that red and yellow rays are also able to 

 act energetically in a chemical manner. 



These fundamental facts may be also demonstrated with the aid of light 

 which has passed through bichromate of potash on the one hand and ammoni- 

 acal copper-oxide on the other. In yellow light photographic paper is only 

 slowly blackened, but assimilation proceeds almost as rapidly under such 

 illumination as in white light ; on the other hand, light which has passed 

 through ammoniacal oxide of copper exerts a vigorous decomposition in silver 

 salts but has little or no effect in assimilation. 



As to the position of the chief maximum point there is much dispute. 

 Reinke considers it to be between Fraunhofer's lines a and B {X = 720-685 fxp), 

 Engelmann and Timiriasef between B and C (A = 685-655 fxfx), Pfeffer 

 (1871) between C and D {X = 655-590 /a/a). It ought to be easy to settle this 

 question, though it is really of minor importance. In addition to the difficulties 



zx 



va : a^ :*<? iS!*) jj^ -,■ m tso\ 

 a B C b e'b f (1 h 



Fig. 27. Curve of evolution of gas-bubbles, 

 compared with the absorption-spectrum of 

 a living leaf. (After Reinke, Bot. Ztg. 1884, 

 pi. I, fig. 6.) 



Fig. 28. Assimilation (firm line) and 

 absorption (dotted line) in green cells be- 

 tween wave lengths A =420-750. (After 

 Engelmann, Bot. Ztg. 1884, pi. "ll, fig. i.) 



which have been already alluded to (e. g. the attainment of a sufficiently pure 

 spectrum with the necessary light intensity), one fact must be emphasized which 

 Engelmann has drawn attention to and which Pfeffer has more recently 

 (Physiol. 2nd ed. I. § 60) brought prominently forward. Chlorophyll has very 

 different powers of absorbing light of different colours. The deepest absorption 

 band of chlorophyll lies immediately in the vicinity of the assimilation maximum 

 (Fig. 24), i. e. near C (X = 661). Although this vigorously absorbed light has 

 also the greatest effect in assimilation, still it can only be fully effective on 

 structures of limited thickness. If we employ an ordinary leaf blade for experi- 

 ment, the uppermost layers containing chlorophyll will absorb all the light of 

 wave length about 660, and the layers lying below will be in darkness. Rays 

 outside this limited region, e. g. those whose A = 630, will be much less absorbed 

 and will pass more deeply into the leaf, and will be able eventually to exert 

 greater assimilatory activity than those which owing to absorption are rapidly 

 altered. Theoretically the more interesting assimilating curve {primary curve) 

 is naturally that investigated by Engelmann, not that actually observed on 

 using thicker leaves. Even in layers of chlorophyll of limited thickness this 

 primary curve is masked; the following values which Engelmann obtained 

 from a comparison of the directly illuminated side and the reverse side of 

 a cell of Cladophora, only 0-028 mm. thick, shows this clearly : — 



