PHO TOS YN THESIS. 



51. Is light absolutely essential to Photosynthesis? 

 Answer by Experiment 24. 



EXPKRIMKNT 24. This may be tested by placing similar leaves for 

 a time under conditions precisely alike except that one is exposed to 

 light and the other kept in darkness, and then applying a test for 

 starch. Place a Tropaeolum or other convenient thin-leaved plant 

 for two nights and a day in darkness (to empty it of starch) ; as early 

 as convenient the next day bring it into light ; cover one leaf com- 

 pletely on both sides with tin-foil, leaving another beside it uncov- 

 ered ; or, cover a leaf with tin-foil in which, matching above and 

 below, some pattern has been cut (see Fig. 16), leaving the leaf there 

 exposed to light on both faces; or, by means of pins thrust through 



FIG. 16. ARRANGEMENTS FOR STUDY OF PHOTOSYNTHESIS. (Instead of the black 

 paper cover on the left, tin-foil should be used.) One-third the true size. 



the leaf, place two pieces of cork on opposite sides of a leaf, match- 

 ing above and below (see Fig. 16). Keep the plant all day in 

 bright sunlight, and towards sunset place the leaves for a few min- 

 utes in boiling water (to kill them and swell the starch), and place 

 in 70.^ alcohol. Later, at a convenient time, warm the alcohol in a 

 water-bath and renew it until all the green is removed, and the leaves 

 are blanched. To bring out the starch distribution, place them in a 



