90 PLANT PHYSIOLOGY 



faces of the leaf to serve as a light screen, the resultant white circle in the 

 iodine-treated leaf being attributed to alsence of light. As a rratter of fact 

 this inference is fallacious, as any control experirrent would show, for the 

 result is almost equally due to absence of the necessary carbon dioxide. This 

 has been shown by Miss Hatjg in the Botanical Gazette, 36, 1903, 389, and 

 by C. A. King in Torreya, 5, 1905, 67, and the Plant World, 8, 1905, 263. The 

 corks do not entirely exclude this necessary gas, for some can leak between 

 cork and leaf, possibly a little through the cork, and a trifle from neighbor- 

 ing intercellular spaces; but they exclude it from all the central part; and 

 the leaf remains just as white, except for a shaded margin, if the cork be 

 replaced by a suitable control arrangement of cork and glass. The experi- 

 ment was used originally by Sachs to show translocation of starch from 

 unlighted areas, and here it is quite correct, but since his time it has been 

 erroneously applied by many. 



The student is here in contact with one of the most significant 

 facts in all organic nature, viz., the appearance of starch in 

 lighted green leaves. He may care, therefore, to experiment 

 with it somewhat further, in which case he will be aided by the 

 following suggestions: 



Suggested Experiments. Microscopic Observation oj the Action oj 

 Iodine on Starch. Cut sections of certain leaves, then blanch and clear them 

 either by strong chloral hydrate, or by strong caustic potash (afterwards 

 washed put); mount the sections in water, observing them with the micro- 

 scope, and apply iodine solution. Directions for accomplishing the whole 

 process in the neatest manner are given by Detmer, 46. 



Relation between Intensity oj Light and Amount oj Starch. This may be 

 well shown by using a screen composed of narrow strips of thin translucent 

 paper, arranged in thicknesses of one, two, three, etc., thus varying the inten- 

 sity of the light without altering its quality. 



But it may be shown far more strikingly by the method recommended 

 by Gardiner (Annals of Botany, 4, 1889, 163), in which the screen is formed 

 by a photographic negative. In this way a positive photograph may be 

 printed in starch on the leaf, and "developed" by iodine. It is essential 

 that the negative be held pressed flat against the leaf, which is well and con- 

 veniently accomplished by the normal light screens earlier described (page 

 87); and it is also better to select a naturally flat leaf, such as Abutilon or 

 Phaseolus vulgaris (String Bean). 



Time Required jor the Appearance oj Starch. This may be determined 

 effectively as follows: Place a plant for one or two days in the dark (when 

 it empties itself of starch by translocation, as will later be noted); bring it 

 into bright light, keeping it at a temperature of 2o°-22°; take from it at 

 regular- five-minute intervals small discs of tissue, and mark them; later 

 blanch these and apply to them the iodine test. The leaf-area cutter, later 

 described, is very convenient for this, but a cork -borer working against a 

 cork is also' good. 



