THE ACTION OF LIGHT 



563 



Engelmann's results, it appears that, in proportion to the light absorbed, the 

 efficiency of the various regions of absorption is the same. In other words, so long 

 as light is absorbed, it does not seem to matter what the wave length is. Kniep 

 and Minder (1909), also, have compared the carbon assimilation with the relative 

 amount of energy of the light absorbed in different parts of the spectrum and state 

 that it is in direct proportion. This fact seems to suggest that the actual pigment 

 itself is merely an optical sensitiser, since there 

 is no relation between its particular absorption 

 bands and the photo-chemical change. 



Lasareff (1907) similarly showed that the bleach- 

 ing of certain dyes is in proportion to the light energy 

 absorbed, whatever the colour of the light. It is 

 obvious, however, that, in so complex a system as 

 the living cell, an exact agreement is not to be ex- 

 pected ; the oxygen, for example, may be partly used 

 up by the protoplasm, and structures other than the 

 chloroplasts absorb light. 



We may take it, then, that the maximal 

 effect of the chlorophyll system is in relation to 

 that part of the spectrum which is absorbed 

 most. 



THE STRUCTURE OF THE 

 CHLOROPLAST 



In view of the results obtained by various 

 observers with solutions of chlorophyll ex- 

 tracted from the leaf, it is important to 

 remember that, in situ, this pigment is closely 

 associated with other substances in the granules 

 known as chloroplasts. It appears to form a 

 thin, highly concentrated layer on the surface 

 of these bodies and is practically solid (see the 

 paper by Timiriazeff, 1903, p. 455), or in the 

 colloidal state, since it shows no fluorescence. 

 As we saw (page 562), its spectrum in the leaf 

 is the same as that of the colloidal solution. 

 Owing to its close association with the complex 

 system of the chloroplast, it is scarcely to be 

 expected that it would be possible to obtain 

 the complete photo-chemical change in pre- 

 parations containing chlorophyll alone. Miss 

 Irving (1910) found that, if a seedling be 

 grown in the dark and then placed in light, 

 chlorophyll may be produced in the cells before 

 they have developed the power of photo- 

 synthesis. At the same time, it is obviously 

 of interest and importance to commence with 

 the action of pure chlorophyll and, if possible, 

 add the other constituents of the system later. 



In this connection, we may remember the importance of the structure of the 

 cell, not only for oxidation processes, about which we shall have to speak later, but 

 also for the re-establishment of lactic acid in the contractile system of muscle 

 with addition of energy, a process more analogous to that of photo-synthesis. 



THE PHOTO-CHEMICAL REACTIONS OF THE CHLOROPHYLL 



SYSTEM 



The final result of the process may be represented by an equation such as : 



a!O 2 + a;H 2 O + light energy = C^H^O^ + aO 2 , 

 but this naturally gives us no indication as to how it is brought about. 



J 



to -T - ' !, 

 . 3* 3j 



FIG. 180. PRODUCTION OF STARCH 

 IN THE SPECTRUM. 



Hydrangea leaves, still attached to the plant, 

 have been deprived of starch by keeping 

 in the dark. They have then projected upon 

 them a small solar spectrum for five to six 

 hours. Subsequent treatment with iodine, 

 in the usual way, shows a picture of the 

 absorption spectrum of chlorophyll in the 

 blue "compound" of iodine and starch. 

 The lower piece of leaf has been partiallv 

 covered with a screen, represented below 

 it, in such a way that the wider part of 

 the aperture corresponded with the region 

 of the spectrum between the lines B and C. 



(Timiriazeff; 1903, p. 434.) 



