Nutrition. 



95 



the leaves. The solution should be of such concentration 

 as to appear a dark wine color. The portions containing 

 starch should be stained a dark blue color by the iodide, and 

 the relation of the action of chlorophyl to light will be indi- 

 cated by the results. Repeat the test with single leaves in 

 reddish and blue light, as in §94, and ascertain what rays are 

 concerned in the food formation. 



Starch is not the direct product of the activity of the chloro- 

 phyl in photosynthesis. Some form of sugar is probably built 

 up in the leaf, and when it begins to accumulate or reaches a 

 certain concentration in the sap, some of it is condensed into 

 starch. Starch may be taken to indicate an accumulation of 

 the products of the photosynthetic activity of chlorophyl. 



98. Relation of stomata to food formation. — Pro- 

 cure a living specimen of some species, with the upper sur- 

 faces of the leaves free 

 from stomata, and 

 place in dark room 

 until the leaves are 

 shown to be free from 

 starch. Now place in 

 the light. Cover the 

 entire under side of 

 one leaf with warm 

 cocoa butter in such 

 manner as to close 



all of the stomata. Treat one-half of the lower side of a 

 second leaf in the same manner. Cover the entire lower side 

 of a third leaf with the butter, and then carefully cut through 

 the upper epidermis in three or four long lines. After expos- 

 ure to sunlight for a few hours, remove one of the treated 

 leaves and one of the normal, extract the chlorophyl and 

 test with iodine, as in §97. Is the leaf able to carrry on 

 sufficient food formation to accumulate a surplus which is 



^^^v 



Fig. 67.— Leaf after treatment with iodine. 

 The stomata of the unshaded portion have been 

 closed with cocoa butter while exposed to light. 

 After Stahl. 



