94 



Elementary Plant Physiology. 



Fig. 65. Method of exclusion of light 

 from portion of a leaf. 



and that oxygen enough to support combustion has been 

 given off. The test may be repeated a number of times. 

 97. Formation of starch in light. Place a good 



specimen of some plant with 

 thin leaves in darkness for a 

 day or two, until but little 

 starch can be detected in the 

 leaves by the use of iodine 

 solution. Bring the plant out 

 into light, and cover one leaf 

 completely with aluminum or 

 tin-foil. Place a thin section 

 of cork on the lower side of 

 another leaf, and a second 

 exactly opposite it on the 



upper side, and drive a pin through both corks and the leaf 

 to hold them together. After the plant has been exposed to 

 bright sunlight for four to six 

 hours, take an untreated leaf, 

 the one that has been covered 

 with tin-foil, and the one to 

 which the corks have been 

 fastened, and plunge into 

 boiling water for a few min- 

 utes. Next place in a porce- 

 lain dish containing alcohol 

 (75 per cent.) and warm 

 gently, renewing the alcohol 

 from time to time until all of 

 the chlorophyl is extracted. 

 Remove all the alcohol, and 

 wash with distilled water 



poured over the leaves in the dish. Pour off the water, add 

 enough of a solution of iodine in potassium iodide to cover 



Fig. 66. Leaf treated with iodine 

 solution, showing uncolored portion 

 which had been darkened in Fig. 65. 



