144 AGRICULTURAL BOTANY 



Note. Take a green leaf from a plant at 6 o'clock in the after- 

 noon. Soak it in alcohol to remove the chlorophyll, then put it inlo 

 iodine and note that it turns dark purple, showing that starch is present. 

 Take another leaf oft" the same plant at 5 o'clock in the morning and 

 give it the same test, and no starch is revealed. The starch has been 

 changed to sugar. 



The Flow of Plant Food Downward. --This food, 

 prepared almost wholly in the leaves of plants, must 

 also pass to other parts of the plant if they are to grow. 

 There is, therefore, in all growing plants a movement 

 of the prepared food from the leaves to the stems and 

 roots. To demonstrate this, cut a little notch through 

 the bark and a little into the cambium layer of a tree. 

 The uninjured cells around the edges of the cambium 

 layer on the upper side of the notch will form a new layer 

 of cells, but not on the lower side, thus showing that 

 the cell food is coming from above. In exogenous 

 plants the downward flow of plant food is through the 

 inner layers of the bark. 



Girdling Trees. Because of this downward flow 

 of the prepared food and upward flow of cell sap, it is 

 easy to see what the effect would be of cutting a notch 

 through the bark of a tree completely around it. The 

 downward current of food prepared by the leaves would 

 be checked and the roots would die for lack of food. 

 If this is done after the unfolding of the leaves in the 

 spring, they may remain green for some time because, as 

 we have learned, the cell sap passes upward through the 

 uninjured sapwood, so that the leaves receive nourish- 

 ment for some time. If, however, the notch is cut 

 through the sapwood also, the whole tree will soon die. 



Food Storage and Uses. Not all the food prepared 

 by the plant is used in growth. The surplus is stored 



