86 PLANT PHYSIOLOGY. 



porcelain or other white-bottomed dish, and pour over them an 

 iodine solution (made by dissolving solid iodine in weak aqueous 

 solution of potassic iodide until the mixture has a dark wine color), 

 which will turn all starch dark blue. 



(In warming the leaf in alcohol to blanch it more rapidly, tlie 

 alcohol should be placed in a porcelain dish in a water-bath ; and the 

 flame must not be allowed to reach //.) 



For the great majority of plants, perhaps for all, the preliminary day 

 or more in darkness is indispensable to the success of the experiment. 

 When tried out of doors it is not enough to simply enclose the leaves in 

 a black bag or box unless this is also shaded, as the heating in sunlight 

 injures the parts. Doubtless the best way would be to run the branch 

 into a double-walled dark box. Particularly good leaves for the ex- 

 periment are cucumber and Abutilon. Thick leaves like those of Ficus 

 are not so good, since it takes long to empty them of starch. 



Of much value is experimentation to prove the increase in dry weight 

 through Photosynthesis. There is, however, no easy method of accom- 

 plishing it, and it hardly seems profitable to take the time necessary for 

 the very exact weighings, though these have considerable educative value. 

 Such weighings, moreover, have only a qualitative value, since the loss 

 by respiration cannot be determined. The best known method, that of 

 Sachs, is described in Darwin and Acton, 31; anoiher method is given by 

 Detmer, 9. In place of the corn plant used by him I have used with 

 some success Chinese lily-bulbs grown in water, though the possibility 

 of error is here greater than in the seed method. 



52. Is amount of Photosynthesis proportional to amount of sun- 

 light ? 



Answer by an experiment (to be called Experiment 25) 

 invented by yourself. 



(Remember tJie goldoi rule of Rxperiment, i.e., cliatige 



only ii single condition.} 



The very important question now arises, why is Chlorophyll 

 green ? Since its work is dependent upon light, and since 

 sunlight includes many rays of different properties, it would 

 seem probable that the chlorophyll color must be connected 

 with some peculiarity of the nature of light in correlation with 

 the nature of the chlorophyll work. To test this, we must first 

 find out 



53. What effect is produced upon sunlight by its passage 

 through chlorophyll? 



