THE FOOD OF PLANTS. 29 



the entrance of mixed white light it is convenient to place the 

 bell-glasses on plates containing sand. The research material 

 placed under them then receives only mixed yellow, or mixed blue, 

 light. I also frequently used in experiments on the influence on 

 physiological processes of rays differing in refrangibility, card- 

 board boxes pasted inside and outside with dull black paper, 

 having for the back wall a well-fitting lid, while the front one is 

 provided with a large hole. In front of this hole are fixed, in a 

 suitable manner, glass bottles with parallel walls, which contain 

 the coloured fluids (see Fig. 10). The solutions of Potassium 

 bichromate, and ammoniacal Copper oxide contained in the 

 double-walled bell-glasses, or in the bottles just mentioned, a/re 

 submitted to spectroscopic examination, the tube of the spectro- 

 scope being introduced into the bell in the one case, the bottle 

 being brought close up to the slit of the spectroscope in the other. 

 The solutions must be of such strength as in the one case to allow 

 only the less refrangible rays, as above indicated, to pass ; in the 

 other, only the more refrangible rays. 



If now crude chlorophyll solutions are exposed to the influence 

 of mixed yellow and mixed blue light (direct sunlight), it is 

 found that the less refrangible rays bring about the decomposition 

 (change of colour) of the chlorophyll far more rapidly than the 

 more refrangible rays. The so-called chemical rays, therefore, 

 i.e. those which are able to decompose Silver chloride, participate 

 only in a subordinate degree in the decomposition of chlorophyll, 

 since the mixed blue light is rich in these chemical rays, while 

 the mixed yellow light is poor in them, as we may readily 

 ascertain by exposing photographic paper to the two kinds of 



1 See Sachs, Botan. Zeitung, 1864, p. 38. 



2 See Pringsheim's Jahrbiicher, Bd. 12. 



3 For further literature, see Detmer, Lehrbuch d. Pflanzenpliysiologie, 

 1883, p. 18. 



9. The Autumnal Colouring of Leaves, and the Winter Colour- 

 ing of Persistent Plant Structures. 



Many leaves turn red in autumn before falling. This pheno- 

 menon may be particularly well studied in the leaves of species 

 of Rhus, as also in those of Cornus sanguinea and Anipelopsis 



