66 ELEMENTS OF PLANTS. 



that of wood sorrel (oxalis,) oxalic ; that of grape vines 

 and grapes, tartaric acid. 



234. All these unite with the oxides of the metals 

 that have been spoken of, and one or more of the salts 

 formed by the union are found in the cells or at least in 

 the ashes of nearly all plants. The salts of potassa, for 

 example, are always found in the ashes of potatoes, tur 

 nips, the grape vine and many others ; and none of these 

 plants can flourish in a soil, however rich in other respects, 

 which contains no potash. Hence potatoes, turnips, beets, 

 and Indian corn, are sometimes called Potash Plants. 



235. In like manner oats, wheat, barley and rye are 

 called Silica Plants, because the ashes of the straw of 

 these plants are more than half made up of silica. And 

 because tobacco, pea-straw, clover, and potato-tops, leave 

 ashes of which more than one half is lime, these plants 

 are called Lime Plants. 



236. Phosphates of Lime and Magnesia, in small quan 

 tities, are found in the ashes of all common plants ; but 

 they form from one half to three fourths of the ashes of 

 wheat, and a very large portion of the ashes of other 

 grains. 



237. What then are the most essential elements in the 

 growth of plants ? All plants, without exception, require 

 for their subsistence and nutrition, the atmospheric ele 

 ments, oxygen, nitrogen, hydrogen, and carbon, and the 

 earthy elements, phosphorus, sulphur, potash, lime, mag 

 nesia, and iron. Plants of certain families require silica. 

 Others require common salt, soda, iodides and bromides. 



238. Besides these, three metals, aluminum, man 

 ganese, and copper, arc found very rarely, as oxides, or 

 us sails, in tin; ashes of a few plants ; and, still more rarely, 

 Fluorine, a powerful gas, remarkable for its power to 



