100 CIRCULATION AND USES OF FOOD BY PLANTS 



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) w r ill 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, 

 Experiment to show that one with starch and water, the other with 

 food material passes SU p- ar a nd water, and a piece of parchment 



down in the inner bark. . . , i c i mi 



paper is tied over the end 01 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-osmosis 



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



