330 ELEMENTS OF GENERAL SCIENCE 



olive oil are familiar examples. There is some oil to be found 

 in almost every plant. 



Proteins are the materials of which the living substance 

 is composed. They contain the same three simple substances 

 as the carbohydrates (C, H, O), and also nitrogen, with 

 (usually) sulphur, phosphorus, and other substances. They 

 are the most complex chemical compounds and are not well 

 understood. Proteins are most familiar to us in the form of 

 the white of egg, cheese, and lean meat. 



The mineral salts, such as common table salt, are inorganic 

 chemical compounds. Many of them are of little importance as 

 food materials, and they are never present in large quantities. 



332. Where food is used. Since food is necessary in a 

 growing 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, and since food is needed by all cells for the process 

 of respiration, food will be needed in all living parts. This 

 means, of course, that there will be need of food to furnish 

 energy everywhere 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. 



333. Method of transfer. Liquids, and solids dissolved in 

 liquids, are able to pass readily from cell to cell, 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 conies to 



