86 PLANT PHYSIOLOGY 



ously disturbed in the performance of their usual functions, and are 

 as closely alike to start with as possible, for which latter reason they 

 should be parts of the same plant, or, better yet, parts of the same 

 leaf. 



EXPERIMENT. Select a nearly grown leaf from the best of the 

 materials mentioned below, and, in the early morning, place on it a 

 normal light screen; leave the plant in bright, but not intense, light 

 for a full day, and towards evening examine it for any observable 

 macroscopic or microscopic differences between the lighted and the 

 darkened portions. Then blanch it by removing the chlorophyl as 

 described earlier (page 79), and examine again. Then apply the 

 first of all microchemical tests, a solution of iodine, and explain 

 the visible result. 



PRECAUTIONS. In order that the screen may be attached early to the 

 plant, it may be put in place the evening before. In practice, however, much 

 more certain results are obtained with most plants if they are first placed for 

 a day (or two if it is cool) in a dark chamber, in which case the screen need 

 remain attached to the plant only an hour, or even less, after it is brought 

 into the light. The plant should never be stood in direct sunlight, which 

 might burn the screened leaf, and the temperature should not be allowed 

 to rise above about 22 (about 20 is best) for reasons which will appear 

 later under Translocation. Also with most leaves the blanching is quicker 

 and the result is plainer if the leaf is placed in boiling water for a minute or 

 two when removed from the plant. If the leaf is to be handled, the alcohol 

 should be replaced by water for a minute or two to remove the brittleness 

 the alcohol causes. The leaf blanches much better if flat or loosely rolled, 

 since crumpling or folding prevents free circulation of the alcohol. In using 

 the test with some thick leaves it will be found that they yield the chlorophyl 

 slowly; the blanching may be hastened by addition of a little caustic potash 

 to the water. Sometimes, also, because of the presence of brown coloring- 

 matters, they show the iodine-blue but badly. Hence it is best to use thin 

 soft leaves. 



MATERIALS. These have been investigated from this point of view in 

 my laboratory by Miss ECKERSON, whose paper upon the subject is expected 

 to appear later, probably in the Botanical Gazette. She finds that the best 

 leaves for yielding a marked and rapid response in this experiment are, in 

 approximate order of excellence, Sunflower, Fuchsia, String Bean, Horseshoe 

 Geranium, Squash, Castor Bean, Indian Com, and seedlings of most com- 

 mon plants. All in this list, if previously kept for a day or two in darkness, 

 will show some starch in from 15 to 20 minutes, and an abundance of starch 

 within an hour. The Fuchsia, Sunflower, Castor Bean, and most seedlings 

 ordinarily need not be kept in darkness longer than one night and the mom- 

 ing up to the time of beginning the experiment. 



Compare also the important list given by MEYER in Botanische Zeitung, 

 34, 1885, 417 (synopsis in JOST, in). 



NORMAL LIGHT SCREENS. It is quite wrong, in good experimenting, to 



