32 PHYSIOLOGY OF NUTRITION. 



ground, the hypocotyl then straightens, and germination is 

 complete. The especially striking fact is that the cotyledons 

 are green. I have frequently convinced myself that this greening 

 of the Pinus cotyledons takes place in a dark space in which 

 wheat seedlings do not grow green, but only develop a yellow 

 plumule. 



Seedlings of monocotyledons and dicotyledons (Phaseolus, Pisum, 

 Raphanus, Triticum, Zea, etc.) grown in the dark, not only grow 

 green when exposed to comparatively bright light ; the formation 

 of chlorophyll also takes place even in greatly weakened light, as 

 we can readily ascertain by placing the objects at the back of a 

 room, and here suitably screening them. We may further cause 

 plants to become green by artificial light. Thus, for example, I 

 placed wheat seedlings grown in the dark, and with plumules 

 2-3 cm. long, at a distance of 15 cm. from the flame of a petroleum 

 lamp. The seedlings were in a crystallising glass with a little 

 water. They became distinctly green, when thus illuminated by 

 the light from the lamp, in a few hours ; control plants, which 

 were kept in the dark, did not become green. 



It is further instructive to study the influence on chlorophyll 

 formation of rays differing in refrangibility. For this purpose 

 wheat seedlings, for example, raised in the dark, and with plumules 

 about 2 cm. long, are placed in small glass dishes with a little 

 water. These, or even small flower-pots, filled with soil, in which 

 the seedlings have been raised, are now placed under double-walled 

 bell-glasses, one of which is filled with a solution of Potassium 

 bichromate, while the other contains an ammoniacal solution of 

 Copper oxide (see 8). We can easily ascertain, by placing under the 

 bells at the same time strips of photographic paper, that the mixed 

 yellow light is almost completely free from the so-called chemical 

 rays, while the light transmitted by the ammoniacal Copper oxide 

 solution very rapidly brings about the decomposition of the Silver- 

 chloride. Having in the morning of a cloudy November day 

 placed the apparatus at a distance of 5 m. from the window, in a 

 room with a south aspect, at a temperature of about 20 C., I 

 found at the end of twenty-four hours that the plumules of the 

 seedlings under the influence of the mixed yellow light had become 

 thoroughly green, while those which had been exposed to the 

 action of the mixed blue light had a plumule light green in colour. 

 If, on the other hand, we expose the apparatus to direct sunlight, 

 the greening takes place more rapidly in the mixed blue light than 



