PLANT MANUFACTURE OF FOOD 139 



without the addition of any other elements. Previous to this 

 knowledge of plant physiology, men believed that the bulk of a 

 plant was derived from materials taken out of the soil. But about 

 three hundred years ago an interesting experiment was carried 

 out which proved the falsity of this idea. Carefully weighed and 

 dried soil was placed in a pot, and in this was planted a willow 

 weighing five pounds. The pot was watered with distilled water 

 and covered to prevent accurnulation of dust. Five years later the 

 willow was removed and found to weigh 169 pounds; but when 

 the soil in the pot was dried and weighed again it was found to 

 have lost only two ounces of its previous weight! Thus it was 

 proven that plants extract most of the material for their bodies 

 from sources other than the soil. We know today that these 

 sources are the carbon dioxide of the air, and water. 



It has long been known that plants remove some substances 

 from the soil; only two ounces of soil were used up by the willow 

 in the experiment just described, but the content of those two 

 ounces is very significant. Continued growing of plants in a cer- 

 tain amount of soil will eventually remove certain substances 

 essential for the life of the plant and for the manufacture of neces- 

 sary plant foods. In nature such a condition rarely occurs since 

 all the plants in an area eventually die and their bodies return 

 to the soil what was removed during their life. Early man solved 

 this problem of the disappearance of soil fertility by moving from 

 one region to another after his crops had exhausted the soil. 



Then man discovered that he could farm an area almost 

 indefinitely provided he returned to the soil each season what the 

 previous season's crop had removed. This made possible con- 

 tinued farming of a region by preserving its soil fertility. The 

 importance of crop rotation and leguminous plants, in this 

 respect, has already been noted; as has the importance of decay 

 bacteria. Manure, wood ash and saltpeter were known to renew 

 this fertiUty; though little was known of why this was so. 



When the chemical nature of plant foods became known, it 

 was possible to state which elements had to be withdrawn from 

 the soil by plants in order to have such compounds produced. 

 This was very evident in the nature of proteins; since these foods 

 contained nitrogen and sulphur, the two elements must of 



