Study of Spirogyra nitida. 251 



dioxide and water and causing their subsequent combina- 

 tion to form organic products. These writers argue, that 

 only the rays which are absorbed are capable of causing 

 chemical changes in the plant, and therefore the absorption 

 bauds of the chlorophyll spectrum correspond to the regions 

 in which growth takes place. 



Pringsheim, taking exactly the opposite view, considers 

 that the rays absorbed are those which would be prejudicial 

 to the growth of the plant, and that they are in this way 

 removed. Pfeffer believes, that the yellow rays — and there 

 is no absorption of these — are those most active in assimi- 

 lation. But, the conditions under which he worked beiiu: 

 inexact, too much reliance cannot be placed upon his de- 

 ductions. 



Engelmann's experiments tend to confirm the work of 

 the school of TimiriazefF, namely, that the region of most 

 active assimilation is situated at the junction of the red and 

 orange, which region is also that of the densest absorption 

 band in the chlorophyll spectrum. 



The theories regarding the action of light have been 

 based upon the rate of decomposition of carbon dioxide 

 and upon the corresponding evolution of oxygen as deter- 

 mined for the different parts of the spectrum. The results 

 obtained with Spirogyra show the general condition of the 

 cells, and the changes in quantity and in kind of the sub- 

 stances produced, by the growth of the plant. 



The spectrum of living chlorophyll as determined by 

 Mann* gives four absorption bands. The first two, which 

 extend from 678 pp to 662 pp, and from 654^// to 638 fj.pi, are 

 both included in the band furnished by the red screen, which 

 lies between 718 pp and 629 pp. What are commonly 

 known as the third and fourth bands of Kraus, Mann does 

 not find in Spirogyra, but the fifth he places between 531 

 pp and 459 pp. This band is partly covered by the blue 

 screen (from 476 pp to 435 pp.) and a very small portion is 

 covered by the green (535 pp to 510 pp.) The sixth band 



*Proc. Bot. Soc. of Edinburgh, Vol. 18, (1890.) 

 17 



