100 CIRCULATION AND USES OF FOOD BY PLANTS 



Experiment to show that 

 food material passes 

 down in the inner bark. 



less visitors. In the same manner mice and other gnawing ani- 

 mals kill fruit trees. Food substances are also conducted to a 



much less extent in the wood itself, and food 

 passes from the inner bark to the center of 

 the tree by way of the pith plates. This can 

 be proved by testing for starch in the pith 

 plates of young stems. It is found that 

 much starch is stored in this part of the tree 

 trunk. 



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 process of osmosis. This is 

 shown in the experiment illustrated in the 

 figure. Two thistle tubes are partly filled, 

 one with starch and water, the other with 

 sugar and water, and a piece of parchment 

 paper is tied over the end of each. The 



lower ends of both tubes are placed in a glass dish under water. 



After twenty-four hours, the water in the dish is tested for starch, 



and then for sugar. We find that only the sugar, which has been 



dissolved by the water, can pass 



through the membrane. 



Digestion. — Much of the food 



made in the leaves is stored 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, for otherwise it could not ~' 



pass through the delicate cell 



membranes. This is accomplished 



by the process of digestion. We 



have already seen that starch is 



changed to grape sugar in the Experiment to show osmosis of sugar 



. (right hand tube) and non-osmoses 



corn by the action of a substance of starch (left hand tube). 



