2S6 The Plant World. 



of non-nitrogenous food, that is, with the manufacture of sugar, 

 is an interference with an early and a fundamental stage in the 

 process of nutrition. This interference stops, or at least propor- 

 tionally decreases, all of the other numerous processes depend- 

 ant upon it. 



These speculations as to the interference of a thin but opa- 

 que coat of cement with light absorption and with the manufac- 

 ture of food may readily be converted into proof. If one gently 

 washes the coating of cement from one side of the midrib, from 

 base to tip, of the upper surface of an orange leaf, without dis- 

 turbing the coating on the other side, and taking pains that the 

 process of cleaning the leaf does not injure it or its attachment 

 to the branch, the leaf remaining undisturbed upon the tree 

 throughout the following day , one will be able to see bya comparison 

 of the amounts of starch in the cleaned and uncleaned halves of 

 the leaf, how much food has been made in each. 



This can be done by making sections of the two halves of 

 the leaf, decolorizing and staining with iodine, clearing, and 

 counting the number of starch grains in individual cells; or one 

 may even attain the same results, although not with the same 

 accuracy, by decolorizing the unsectioned halves of the leaves, 

 treating them with iodine, and then clearing again by the well- 

 known method of Sachs. 



Since the starch made during the day is removed from the 

 orange leaf during the night, and the starch found in an orange 

 leaf at the end of the daylight hours of the day represents the 

 amount which has accumulated in its cells since morning, a com- 

 parison of the cleaned and coated halves of the leaves at the end 

 of the day, is not only fair but instructive, it shows what the cells 

 in these two halves have been able to do under conditions which 

 differ only in the one respect, that the entrance of light into 

 the cells on one side of the midrib has been interfered with, or 

 prevented, and on the other side has not. Counting the grains 

 of starch in mesophyll cells in sections through the washed and 

 through the unwashed halves of orange leaves shows that the 

 cells in the washed half contain four or five times as many grains 

 of starch as those in the covered and shaded half. 



Here, then, is the proof that food manufacture at the very 

 beginning of the series of processes which we collectively call 

 nutrition, is interfered with. The degree of interference is 



