5 62 



PRINCIPLES OF GENERAL PHYSIOLOGY 



It is isomeric with the " lycoperdin " of the tomato. As we shall see later, it may 

 play a part in the decomposition of carbon dioxide by the chloroplast system. 



Xanthophyll is an oxide of carotin (C 40 H 56 O 2 ). It is insoluble in petroleum 

 ether, but soluble in methyl alcohol. It is isomeric with the " lutein " of the fowl's 

 egg, which has no relation to cholesterol, as had been supposed. 



a B 



ABSORPTION OF LIGHT BY CHLOROPHYLL 



In Fig. 178 the absorption spectrum of chlorophyll, in its two forms, is given. 

 The most striking and characteristic appearance is the dark band in the red, which 

 is divided into two in chlorophyll-6. As will be noted, the chief absorption is in 

 the longer wave lengths and is practically in the position of the maximum energy 

 of the solar spectrum, during the greater part of the day. S. P. Langley's 

 measurements of the position of maximum energy gave 650-666 /M/I for high sun. 

 The latter number is easy to remember, as Timiriazeff points out, being the " number 

 of the beast." The middle of the chief band of chlorophyll-a is, in solution in ether, 



at 662 fifji. In colloidal solution in 1 

 per cent, acetone, the band is shifted 

 towards the red, so that its maximum, 

 is at 678 /I/M. This is the same as that 

 of its natural state in the leaf. The 

 maximum energy of solar radiation, 

 also, would be for the greater part of 

 the day nearer the red than the figure 

 of Langley. Chlorophyll has a con- 

 siderable absorption in the blue also, 

 but practically none in the infra-red, 

 nor in the yellow-green, not much in 

 the ultra-violet. 



It is remarkable that some of the earlier 

 observers believed that their experiments 

 showed that the maximum photo-chemical 

 change occurred in the j'ellow-green region, 

 in which the absorption of light energy is 

 minimal. This would be a difficulty in 

 view of Grotthus's law, and later observa- 

 tions, especially by Engelmann, showed it 

 to be due to incorrect estimation of the 

 absorption of the screens used. 



A striking demonstration of the 

 fact that the maximum evolution of 

 oxygen is at the place of the greatest 

 (Engelmann, 1882, 1, p. 195.) absorption of light was given by 



Engelmann (1882, 1). This was done 



by the use of a bacterium, which was very sensitive to oxygen. Water 

 containing these organisms was placed, along with a thread of a green alga, 

 on the stage of a microscope. In the same plane, and along the thread of 

 alga, a minute spectrum was projected by means of a spectroscopic arrange- 

 ment beneath the stage. It was seen that the bacteria accumulated precisely 

 at the places where the absorption bands of chlorophyll were situated (see 

 Fig. 179). Another experiment, showing the same fact, is due to Timiriazeff 

 (1903). A leaf on a plant is deprived of its stored starch by being kept in the 

 dark. A small spectrum is projected on to its surface and, after some time, the 

 leaf is decolorised by alcohol and treated with iodine. The absorption bands of 

 chlorophyll are then found to be mapped out by the action of the iodine on the 

 starch, which has only been formed in these places (Fig. 180). Measurements have 

 also been made, spectrophotometrically, of the absorption of light in different 

 regions of the spectrum and compared with the photo-chemical effect. There are 

 two maxima shown, but, when the curve is corrected for the normal spectrum, in 

 which equal abscissae correspond to equal differences of wave length, the second 

 maximum in the blue end is found to be comparatively unimportant. From 



FIG. 179. PRODUCTION OF OXYGEN IN POSITION 

 OF MAXIMAL ABSORPTION OF LIGHT BY 

 CHLOROPHYLL. Part of a nlament of Clado- 

 phora in water containing motile bacteria, 

 of considerable avidity for oxygen. 



a, Spectrum of sunlight, indicated by the position of 

 the Fraunhofer lines, is projected from below to lie 

 along the filament. The absorption of light by the 

 chloroplasts, which practically fill the cells, is seen 

 l>etween B and C and at the violet end. The accumula- 

 tion of bacteria at places of absorption, especially in the 

 red, shows that oxygen is being produced there. 

 Magnified 188 times. 



