THE FOOD OF PLANTS. 33 



in the mixed yellow light. The result of the last comparative 

 experiment however is open to doubt, unless very special pre- 

 cautions are taken, for I have satisfied myself that the air under 

 a double- walled bell provided with ammoniacal Copper oxide 

 solution becomes much warmer when exposed to direct sunlight, 

 than the air under a bell-glass filled with Potassium bichromate. 

 But still it certainly cannot be regarded as immaterial as to the 

 result of the experiment whether we expose the seedlings grown in 

 the dark to diffuse light or to direct sunlight. If wheat seedlings 

 with yellow plumules are exposed to diffuse light, and others 

 simultaneously to direct sunlight, the former rapidly become 

 green, e.g., in the course of three hours, while the latter produce 

 normal chlorophyll far more slowly. Now we have seen in 8, 

 that chlorophyll in alcoholic extracts from, green plants very 

 rapidly decomposes (changes colour) in direct sunlight, while 

 diffuse light only very slowly induces changes, and this must be 

 remembered in explaining the facts with which we have just 

 become acquainted. In direct sunlight, whether it acts on the 

 seedlings directly, or after traversing a solution of Potassium 

 bichromate, seedlings become green slowly, since the chlorophyll 

 as it is formed is in great part destroyed again. In diffuse light, 

 as also in direct sunlight which has traversed an ammoniacal 

 solution of Copper oxide, energetic decomposition of chlorophyll 

 cannot take place, and so the chlorophyll formed rapidly accumu- 

 lates in the cells of the seedlings. Feeble diffuse light hardly 

 exerts any decomposing action on chlorophyll, and if seedlings are 

 placed far aAvay from the window under double-walled bell-glasses, 

 they become green more rapidly in mixed yellow light than in 

 mixed blue, since the former exerts almost exclusively its greater 

 chlorophyll-forming power, its chlorophyll-destroying power 

 standing very much in the background. 1 



To learn whether the dark heat rays can bring about the green- 

 ing of seedlings grown in the dark, we place, e.g., yellow-plumuled 

 wheat seedlings in small glasses under double-walled bell-glasses 

 filled with a solution of Iodine in Carbon bisulphide. If suffici- 

 ently strong, such a solution transmits no light rays, but permits 

 the passage of the heat rays. The bell-glasses, surrounded at the 

 bottom with sand, are exposed to direct sunlight or diffuse day- 

 light ; the research objects do not become green. 



In order to study the relation between chlorophyll formation 

 and conditions of temperature, it is convenient to employ, e.g., 



P. P. D 



