UTILIZATION OF FOOD IN THE PLANT 247 



267. Where food is used. Since food is necessary in a grow- 

 ing plant as a supply of material from which new leaves, 

 branches, and roots are constructed, it is evident that much 

 of it will be needed in the more rapidly growing parts. Also, 

 since food is necessary to replace the living substance as it is 

 destroyed in respiration, food will be needed in all living parts. 

 This means, of course, that there will be need of food every- 

 where excepting in a few dead tissues like the dry outer bark. 



Although food is needed everywhere, it is not made every- 

 where. Carbohydrates are made principally in the leaves, but 

 they may be needed in connection with the growth of roots 

 or the development of the fruit or in other parts of the plant. 

 This immediately raises the question of how food may pass 

 from one part of the plant to the other, for very plainly there 

 must be some method of transfer. 



268. The method of transfer. Liquids, and solids dissolved 

 in liquids, are able to pass from cell to cell by osmosis, as we 

 learned when studying the absorption of materials by the roots. 

 It is, of course, perfectly easy to see how sugar may be trans- 

 ferred from one cell to another in this way, for the sugar 

 which is in the plant is dissolved in the sap. By osmosis the 

 sugar may travel from cell to cell in the leaf until it comes 

 to the veins, along which it travels to the stem and so through- 

 out the plant. But what about other food materials ? Most 

 kinds of food material in a plant are not soluble. For instance, 

 if we soak a potato or a quantity of rice in water, the valuable 

 materials in it do not dissolve in the water, else we should 

 find it of more advantage to drink the water than to eat the 

 rice or potato. The fact is that in most foods comparatively 

 little of the carbohydrates, fats, and proteins dissolve, no 

 matter how long we soak the foods in water. In the examples 

 mentioned the most abundant material is starch, and this is 

 quite insoluble. Since food material is usually found in plants 

 in the form of starch and other insoluble compounds, we must 

 ask ourselves how it is that food can be transferred to the 



