84 THE VEGETATIVE FUNCTIONS OF PLANTS 



was planted to corn for 28 years in succession. The yield 

 of corn for the last year was 22 bushels per acre. On a 

 second field, planted for the same length of time with 

 alternate crops of corn and oats, the final yield of corn 

 was 36 bushels; while a third field in which corn, oats, and 

 clover were planted alternately, the final yield of corn 

 was 59 bushels — over twice the yield without rotation with 

 a legume. This increase in yield was due chiefly to the 

 enrichment of the soil in nitrates by the organisms that 

 form tubercles on the roots of the clover. If the entire 

 clover crop (tops and all) is plowed under occasionally, 

 so as to serve as "green manure," the results will be 

 better than when the clover tops are always removed 

 as hay. 



83. Manufacture of Proteins. — Proteins are formed 

 de novo only in living plant cells, by combining the prod- 

 ucts of photosynthesis with the nitrogen supplied in the 

 form of nitrates. Their formation may be favored by 

 light, but unlike the process of photosynthesis, protein- 

 formation does not require either light or chlorophyll. 

 The formation of proteins occurs in large measure in 

 foliage-leaves, but may take place in any living cell. The 

 manner of their formation is not as well understood as is 

 that of carbohydrates. 



84. Fats. — Fats may occur in plant cells in either liquid 

 or solid form. We are most familiar with the liquid fats, 

 such as the various oils derived from plants. Fat occurs 

 in the solid form in the cocoanut. In the tallow-tree 

 (Sapium sebiferum) it is so abundant, as a waxy coating 

 on the seeds, that it is used in eastern Asia for making 

 candles. Fats commonly occur in droplets in the proto- 

 plasm, or as an emulsion in the cell-sap, but the place 



