ii2 PLANT PHYSIOLOGY 



what difficult of application, but directions are contained in the accessible 

 books (Darwin and Acton, 35; Detmer, 36). Another method, in some 

 respects ideal, is that of Timiriazeff (described in Comptes Rendus, no, 

 1346), whereby the solar spectrum itself, held in a fixed position by a helio- 

 stat, is projected directly upon a leaf in a dark chamber, the amount of 

 photosynthate formed being determined by the starch test. The spectrum 

 method has been used by others also (Jost, 126). 



It would seem at first sight that monochromatic screens could be con- 

 structed from plates of colored glass or of gelatin, or of films of collodion on 

 glass; and certainly such screens would be much more convenient in manipu- 

 lation than any liquid colors. But unfortunately the obtainable colors of 

 glass or gelatin are shown by the spectroscope to be of mixed composition; 

 only a few of them (including, however, a good red) transmit their own colors 

 alone, which shows how unsafe a guide to the composition of colors is the 

 eye unassisted by the spectroscope. Further, with such screens it is very 

 difficult to secure not only precisely the right colors, but also just the correct 

 intensity. A range of colors suitable for such screens, or filters, is given 

 by MacDotjgal, 131, by Davenport, 157 (also 158), and by Wood, 

 "Physical Optics," 10. 



The result of the foregoing experiment identifies, though in 

 a somewhat crude way, the energy used in photosynthesis, and 

 shows its general relation to the dark bands of the chlorophyl 

 spectrum. It throws no light, however, upon the exact mode 

 of transformation or application of the energy to the especial 

 work of photosynthesis, viz., the dissociation of the substances 

 concerned. The student should now inform himself upon our 

 present knowledge and suppositions upon the subject; and he 

 should make sure he understands the significance of the energy 

 transformation in photosynthesis, viz., the dissociation of very 

 stable simple substances through kinetic energy of light, and the 

 formation of an unsaturated or oxidizable synthate, with the conse- 

 quent storage of latent or potential energy. He is here dealing 

 with the most important single process in the entire range of 

 the chemistry of organic materials. He should also consider 

 here the proportion of the sunlight energy used in photosynthe- 

 sis, on which there is a most valuable paper by Brown in Nature, 

 71, 1905, 522. 



But there are certain minor points also, — why photosynthesis 

 should occur under light waves having such diverse wave-lengths 

 as red and blue, and whether it is possible to produce photosyn- 



