THE METABOLISM OF PLANTS. 255 



which presents itself in considering Draper's and Pfeffer's 

 view is that, inasmuch as the yellow rays are not absorbed 

 by chlorophyll, it is difficult to see in what way chlorophyll 

 assists in the conversion of these rays into work in the chloro- 

 phyll-corpuscle. This is a difficulty which has never yet been 

 explained, and it is one which appears to be hardly susceptible 

 of any satisfactory physical explanation. The view held by 

 Lommel and by Timiriaseff is, a priori, intelligible, for the 

 rays in question are absorbed by the chlorophyll, and it may 

 be inferred that they are converted into chemical work after 

 absorption. Moreover, the experimental evidence is decisively 

 in favour of the latter view. In his experiments Timiriaseff 

 was careful to work with a pure spectrum, that is, he endea- 

 voured to prevent as far as possible any admixture of the 

 rays of different colours, so that the red portion of his spec- 

 trum consisted almost exclusively of red rays, the yellow of 

 yellow rays, and so on. Draper and Pfeffer clearly worked 

 with impure spectra, for in their experiments the aperture of 

 the slit by which the light was admitted was very wide in 

 order to increase the intensity of the illumination. Hence in 

 their spectra there was a considerable admixture of rays of 

 different colours. Their observations are doubtless accurate 

 enough, but their conclusions as to the relative efficacy of the 

 different rays of the spectrum in the decomposition of carbon 

 dioxide are vitiated by the imperfection of the method by 

 which their data were obtained. Timiriaseffs results have 

 received important confirmation by the ingenious experi- 

 ments of Engelmann, to which reference has been already 

 made (p. 156). 



Engelmann ascertained that certain Schizomycetes (Bacterium- termo, 

 Cohn) only exhibit their movements in the presence of free oxygen ; if a 

 drop of water containing these organisms be examined under the micro- 

 scope, the Bacteria collect especially at the edges of the cover-glass and 

 there continue their movements, and those which remain at some distance 

 from the edges soon cease to move. He observed also that if a cell 

 containing chlorophyll, such as a small Alga, be introduced into such a 

 preparation, the Bacteria collect around it, provided, of course, that the 

 illumination is sufficiently intense to ensure the evolution of oxygen by 

 the chlorophyll. By observing the green Alga in the different regions of 



