206 



A YEAR IN SCIENCE 



and industries can scarcely be estimated. The accom- 

 panying table will be of some value in helping you to 

 appreciate their importance : 



VALUE OF CROPS IN THE UNITED STATES IN 1909* 



*Thirteenth Census of the United States, 1910. 



Other foods manufactured by plants. Other foods, 

 proteins, are manufactured by the plants from the 

 carbohydrates and the various substances which come 

 up from the soil. These mineral substances, which are 

 dissolved in soil water, are called solutes. Proteins con- 

 tain carbon, hydrogen, oxygen, nitrogen, and in addi- 

 tion small quantities of many other elements. These 

 are all combined to form a very complex compound 

 which is more like protoplasm than any other non- 

 living substance. We do not know how plants manu- 

 facture proteins. They appear to add nitrogen and 

 other substances from the soil to the carbohydrates. 

 We have before referred to the fact that it is essential 



