WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 171 



A second cup is provided, so that after an interval of exposure of 

 the plant to light, or to darkness, according to the object of the 

 experiment, an equal number of discs is cut from the same leaves ; 

 these discs are treated like the first set, and the second cup placed 

 with the first in the drying-oven. 



When thoroughly dried, both cups are weighed ; then the weights 

 of the cups (stamped on them, with the letters M and N to distin- 

 guish morning and night, or light and darkness, experiments) are 

 subtracted, giving the dry weights of the two sets of leaves, which 

 are of equal known total area. 



226. The Time required for the Appearance of 

 Starch may be determined by bringing into bright light 

 a plant with starch-freed leaves, and then taking discs 

 from the leaves at intervals of, say, ten minutes ; mark each 

 disc, test with iodine, and note the increase in amount of 

 starch. Also experiment with threads of Spirogyra, kept 

 in darkness until starch-free, then exposed to light in 

 water in a watch-glass ; mount pieces in choral-hydrate- 

 iodine at intervals, and note that about five minutes' ex- 

 posure to bright light may be sufficient for the appearance 

 of some starch, while in about half an hour abundant 

 starch will usually be found. 



227. That only a portion of the photosynthate 

 accumulates in the leaf, when in light, is readily 

 proved. Determine, as directed above, the dry weight 

 of a certain area, say 200 sq. cm., of (1) leaves freed from 

 starch by keeping the plant in darkness ; (2) an equal 

 area of leaves after exposure of the plant to bright light 

 for four hours, say from 10 a.m. to 2 p.m. ; (3) an equal 

 area of leaves from same plant after transferring it to 

 darkness from 2 p.m. to 6 p.m. 



On adding the increase in dry weight during the four 

 hours of light and the decrease in dry weight during the 

 four hours of darkness, we get a rough estimate of the 

 total product of photosynthesis. A large proportion of 

 photosynthate migrates from the leaf and is used up in 

 respiration, even in leaves which accumulate a relatively 

 large amount of starch. Even in decidedly starchy leaves, 

 like those of Sunflower, not more than one-sixth of the 

 photosynthate consists of starch. 



