INFLUENCE OF THE DIFFERENT RAYS OF THE SPECTRUM 343 



obtained by Engelmann for green plants in a direct sun-spectrum is shown 

 in Fig. 51 (Ass. Green], and the assimilatory maximum corresponds to 

 the main absorption band between B-C (cf. curve Abs. Green, and the 

 spectra from leaves and from chlorophyll). There is a resemblance between 

 the curves of assimilation and absorption, but the correspondence is 

 not an exact one, and the difference would be still more marked if 

 the frequently doubted secondary assimilatory maximum between F-G did 

 not exist \ 



The presence of phycoerythrin in the chloroplasts of the Florideae causes 

 the assimilatory maximum to be transposed to beyond D, and the rays between 

 B-C no longer exercise any specially favourable effect (Fig. 51, Ass. Red}. 

 The alteration caused by the presence of phycocyanin (Sect. 53) in blue- 

 green algae is less marked (Fig. 52, Ass. Bl.-Gr.}, and in both cases the 

 curve of absorption undergoes a similar though not precisely corresponding 

 change. 



The most active assimilation is caused in purple bacteria by the infra- 

 red rays of 800 to 900^ wave length (IJA/A = oooi /A), but in rays of 

 loco /xju, Engelmann could no longer detect any evolution of oxygen. 

 Hence the heat-rays given off from a stove do not excite any assimilatory 

 activity in these bacteria, although this power is possessed by the rays of 

 light which pass through a solution of iodine in carbon bisulphide ; in such light 

 on the other hand ordinary green plants evolve just as much carbon dioxide 

 as they do in darkness 2 . The assimilatory curve falls steadily towards the 

 blue, where it ceases, although here the maximal absorption of light occurs. 

 Owing to the feebleness of the evolution of oxygen Engelmann was unable 

 to determine the assimilatory curve with precision, and hence it remains 

 uncertain whether the visible red, where absorption is feeble, excites but 

 little assimilation of carbon dioxide. (See also Sect. 52.) 



The more deeply seated chloroplastids screened behind an outer 

 chlorophyll-layer are exposed to light which has lost two to four times as 

 many of the rays between B and C as of the yellow rays, and hence 

 behind even a thin chlorophyll-layer an assimilatory minimum occurs at 

 the point of maximal absorption (cf. Fig. 51) 3 , while the assimilatory 

 maximum is displaced to the line D \ E, in the neighbourhood of the green. 

 Engelmann found that the maximal evolution of oxygen occurred at this 

 point when he examined the upper surface of a thick Cladophora filament 

 illuminated from beneath, whereas on the lower surface the maximum 

 lay between B and C. For these and other reasons (such as the possibility 

 of slight lateral diffusion, &c.) even the bacterium method only enables an 



1 [That a marked secondary assimilatory maximum does exist is, however, certain. See Kohl, 

 Ber. d. Bot. Ges., 1897, p. in.] 



a Pfeffer, Arb. d. Bot. Inst. in Wiirzburg, 1871, Bd. I, p. 41. 

 3 Engelmann, Bot. Zeitung, 1882, p. 421. 



