THE METABOLISM OF PLANTS. 257 



greater is the intensity of light required to produce the movements of the 

 Bacteria in any given region of the spectrum, or, in other words, to cause 

 an evolution of oxygen. Assuming that the evolution of oxygen is pro- 

 portional to the intensity of the light, that is, to the width of the slit, the 

 relative activity of the various regions of the spectrum will be inversely 

 proportional to the width of the slit. The maximum may be taken 

 as loo. 



The following are some of the results obtained in this way. The 

 observations were made with a prism-spectrum, but they are reduced to 

 the diffraction- spectrum. 



I. Green Cells (Sunlight). 



a Bf C CD D DiE Efc EF F F|G G 

 6-38 loo 81-2 55-1 41-2 36-3 69-9 86-1 80-9 47-2 



II. Brown Cells (Diatoms) (Sunlight). 

 i3'3 94'6 77*o 77'i 100 89-3 79-3 75-9 53-8 36-6 



III. Bluish-green Cells (Oscillatoria, Nostoc] (Sunlight). 

 85*3 967 100 44*4 21*2 



IV. Red Cells (Floridea) (Gaslight, calculated to Sunlight). 

 a BC QD D DE D|E EU E^F F G 



J "9 I 5'4 3 J '8 50'5 100 79*4 62^2 36^5 46*4 19*1 



We learn, then, that the chlorophyll (and the other colour- 

 ing matters above alluded to) absorbs certain rays in different 

 parts of the visible solar spectrum, some to a greater and 

 some to a less extent, and that this absorption is the means 

 by which the kinetic energy of the rays is made available for 

 the work of constructive metabolism. It is of interest to note, 

 as Timiriaseff does, that the maximum absorption of chlo- 

 rophyll coincides exactly with that portion of the spectrum in 

 which, according to Langley, the maximum of energy falls. 

 The whole of the kinetic energy thus absorbed is not trans- 

 formed into work, but still the chlorophyll-corpuscle appears 

 to be a very perfect machine in this respect, for, according to 

 TimiriasefFs calculations, it transforms into work as much as 

 forty per cent, of the absorbed energy. 



In concluding this part of the subject we will briefly con- 

 sider the relation between the intensity of the light and the 

 decomposition of carbon dioxide. It is obvious that there 

 must be a minimum intensity at which this process first 

 v. 17 



