146 ROTATION OF CROPS. 



not come to seed, and withered after blossoming. Vetches reached 

 a height of ten inches, blossomed, and put out pods, but they did 

 not contain any seeds. 



Tobacco sowed in the sand, developed itself at first in the usual 

 way, but from June to October the plants reached the height only 

 of five inches ; they had only four leaves and no stem. 



The analysis of the ashes of these plants, and also the analysis 

 of their seeds, proved that although this sterile sand contained 

 such a small quantity of potash and of soluble constituents, still 

 it had yielded a certain quantity of these, and on this quantity the 

 growth of the leaves and of the stem depended ; but it was im- 

 possible that the plants could come to seed, because the con- 

 stituents necessary for the formation of the seeds were entirely 

 absent. 



Phosphoric acid was detected in the ashes of most of the plants 

 growing in this sand, but its quantity corresponded only to that 

 introduced to the soil in the seeds themselves. In the ashes of 

 the tobacco plant, the seeds of which it is known are so small as 

 to contain scarcely an appreciable quantity of phosphoric acid, 

 not a trace of that acid could be detected. 



What theory distinctly indicated as the cause of the sterility 

 of this sand, the experiments of Wiegmann and Polstorf com- 

 pletely established. They took the same sand and prepared from 

 it an artificial soil by the addition of salts manufactured in a 

 laboratory (see Appendix) ; they then sowed in this soil the same 

 plants, and saw that they flourished in the most luxuriant man- 

 ner. The tobacco became three feet in height, and put forth 

 many leaves ; on the 25th of June it began to blossom, and on the 

 10th of August obtained seeds, from which on the 8th of Septem- 

 ber ripe seed capsules with complete seeds were taken. In ex- 

 actly the same way, barley, oats, buck-wheat, and clover grew 

 luxuriantly, blossomed, and yielded ripe and perfect seeds. 



It is quite certain, that the growth of these plants in the for- 

 merly sterile sand, depended upon the salts added to it. An equa. 

 fertility for all plants is given to this artificial soil by the addition 

 of certain substances which are absolutely necessary for the life 

 of the plants, because they are present in the matured plant, and 

 in its stem, leaves, and seeds. 



