186 



DR. DAUBENY ON THE ROTATION OF CROPS, ETC. 



As the experiments were carried on from 1834 to 1844 inclusive, it may be satis- 

 factory to state in the first instance such of the meteorological characters of these 

 years as may be gathered from the Register kept at the Radcliffe Observatory, rela 

 tive to the mean temperature, or from my own observations made at the Botanic 

 Garden, Oxford, as to the amount of rain. 



The plots of ground set apart for the experiments were not exactly equal in point 

 of size, but their square contents being known, it was easy to reduce the crops 

 to one standard, and that of 100 square feet was selected as the most convenient. 



In reporting to the Society the results, I shall therefore always suppose that re- 

 duction as being made, and shall set down what ought to have been the produce, 

 supposing each plot to have measured exactly 100 square feet. In this statement I 

 will begin with the only case in the whole series to which De Candolle's theory of 

 excretions appears at all applicable ; namely, that in which the plant experimented 

 on was a species of Spurge, the Euphorbia lathyris. 



In 1835 a luxuriant crop of this weed was obtained, amounting to about 18 lbs., 

 but the next year the produce had dwindled almost to nothing, and in 1837, in which 

 fresh plants were introduced, an equal failure took place. 



Nor did any new plants start up in 1838, so that in 1839 the plot was sown with 

 flax, barley, and beans, of all of which I obtained a tolerable yield. 



This experiment therefore might be appealed to in support of De Candolle's 

 views, as it would appear, that excretions had been emitted from the roots of the 

 Euphorbia, which proved injurious to plants of the same species as those from which 

 they had proceeded, but which exerted no such poisonous influence upon others not 

 allied to them in organization; or, if it be objected, that during the course of 1838 

 the excretions might have become so far decomposed as to lose their poisonous cha- 



