104 APPLIED BIOLOGY 



light from other sources than the sun will serve for photo- 

 synthesis; for example, electric lights are sometimes used 

 in greenhouses. The blooming of Easter lilies may be 

 hastened from four to ten days; and lettuce grown within 

 fifty or sixty feet of a 2000-candle-power arc-light, used 

 regularly half of the night, will be ready for market a week 

 or more before plants not so treated. Incandescent electric- 

 and gas-lights have a similar effect, but in lesser degree. 



Another illustration of the effect of light is in the rapid 

 maturing of plants under the influence of the intense 

 light of the short arctic summer. Also, plants in greenhouses 

 in winter do not grow as rapidly in a given number of hours 

 of sunshine as they do in summer when the light is so much 

 more intense. And this is so even when the interior of a 

 greenhouse is constantly at the average summer heat. 



101. Disappearance of Starch from Leaves. If we take 

 any plant which has been standing in sunlight, clip off 

 some leaves, and place in alcohol for later testing to make sure 

 that starch is present, and then set the plant in a dark 

 room or box for a night and take other leaves before light 

 reaches them, there will be no blue color with iodine solu- 

 tion, thus indicating that starch has not remained in the leaves 

 kept in darkness. Where has it gone during the night? 

 There are two answers : 



First, some of the starch has probably been used in the cells 

 of the leaf, either changing to other cell-substances composed 

 of the same elements (e.g., sugar), or combining with the 

 elements brought in water from the soil and forming com- 

 pounds containing, besides the C, H, O of the starch, some 

 N, S, and P, and perhaps other essential elements. The 

 compounds thus formed (containing C, H, O, N, S, P) are 

 called albuminous substances or proteins; and some of these 

 may form some new living matter (protoplasm) in the cells 

 of the leaf. 



The second explanation of the fact that the starch dis- 



