52 PHYSIOLOGY OF NUTRITION. 



walled bell-glasses in small dishes containing spring water, or are 

 put into suitable boxes (see 8), and then exposed to light which 

 has traversed solutions of Potassium bichromate and ammo- 

 niacal Copper oxide. We expose the apparatus to direct sun- 

 light or to diffused light. From time to time, say every ten 

 minutes if Ave experiment with direct sunlight, say every thirty 

 minutes if with diffused daylight, we test filaments of Spirogyra 

 or bud leaves of the Elodea for starch. It is found that just as 

 evolution of Oxygen takes place far more actively in mixed yellow 

 light than in mixed blue light, so also starch formation is brought 

 about by rays of lower refrangibility far more copiously than by 

 rays of higher refrangibility. In direct sunlight, however, the 

 formation of starch in the chlorophyll goes on with tolerable 

 rapidity, even under the influence of the mixed blue light. I 

 found, e.g., that filaments of Spirogyra, at first starch-free, con- 

 tained considerable quantities of starch after exposure for thirty- 

 five minutes, at a high temperature, to sunlight which had 

 traversed the ammoniacal solution of Copper oxide. 



It is instructive to investigate by Sachs' macrochemical method 

 variegated leaves which have been vigorously assimilating. We 

 may use for the purpose leaves of Acer negundo or Sanchezia (the 

 last must be leffc for a fairly long time in the Iodine solution). It 

 is seen that only the green parts of the leaf, not the parts free 

 from chlorophyll, contain starch. 



The following experiment which I made with pot plants of 

 Tropeeolum majus is particularly interesting since it teaches that 

 starch formation in the mesophyll of the leaves is strictly localized, 

 inasmuch as assimilatory activity is only exhibited by those parts 

 of the leaf which are directly struck by the sun's rays, no starch 

 being formed, e.g., in artificially darkened parts of the same organ. 

 Plants of TropEeolum are shaded for two days or more until 

 macroscopic tests show that the mesophyll of the leaf has become 

 free from starch. Then with pins we fasten to the upper and 

 lower surfaces of full-grown leaves, exactly opposite one another, 

 small discs of thick cardboard or felt, so that when the plants are 

 again exposed to the light, only portions of the leaf surface will 

 be illuminated. After some time (in my experiments after a day 

 and a half) we test the partly shaded leaves macrochemically. 

 The mesophyll of the artificially darkened regions of the leaf is 

 starch-free ; only the nerves contain starch. On the contrary the 

 parts of the leaf which have been struck by the rays of light are 



