112 



BOTANY 



food supply is cut off from the protoplasm of the cells in the 

 part of the tree below the cut area. 



In what Form does Food pass through the Stem? — We have 

 already seen that materials in solution (those substances which 

 will dissolve in the water) will pass from cell to cell by the pro- 

 cess of osmosis. 



Experiment. — Partly fill one thistle tube with starch and water, and an- 

 other with sugar and water. Tie over the end of each tube a piece of parch- 

 ment paper. Place both test tubes under water in a dish. After twenty-four 



Experiment showing the osmosis of sugar (right-hand tube) and non-osmosis of starch 



(left-hand tube). 



hours, test the water in the dish for starch, and then for sugar. We find 

 only the sugar, which has been dissolved by the water, can pass through 

 the membrane. 



Digestion. — As we shall see later, the food for a plant is manu- 

 factured in the leaves. Much of this food is in the form of starch. 

 But starch, being insoluble, cannot be passed from cell to cell in 

 a plant. It must be changed to a soluble form. It is changed 

 by a process known as digestion. We have already seen that 

 starch was changed to grape sugar in the corn by the action of a 

 substance (a digestive ferment) called diastase. This process of 

 digestion seemingly may take place in all living parts of the plant, 

 although most of it is done in the leaves. 



