Peteie. — On Vegetable Physiology. 429 



from them to be employed in the formcation of new leaves ; 

 but eventually analysis of the mature plant shows no increase 

 of proteid or nitrogenous substance, and but little increase in 

 total weight, when compared with the seed from which it has 

 grown. ' 



Soon after Boussingault's results were communicated to 

 the scientific world, Lawes, Gilbert, and Pugh, the great 

 English authorities on agricultural research, undertook at 

 their well-known experimental farm at Eothamstead a fresh 

 investigation of this question among others. Their results 

 entirely confirmed Boussingault's conclusion that plants do 

 not directly fix and assimilate the free nitrogen of the air, but 

 derive the whole of their supplies of nitrogen from nitrates 

 and other nitrogenous compounds in the soil, presented to 

 their roots in a state of solution. As a result of later re- 

 searches, Lawes, Gilbert, and Pugh drew attention to a very 

 remarkable fact about the leguminous plants which they had 

 investigated. In the case of most crops the nitrogen which 

 they were found on analysis to contain could be entirely 

 accounted for by the combined nitrogen originally contained 

 in the soil, as shown by analysis of samples, or added to it in 

 the form of manure, or washed down from the air by rain. 

 But in particular leguminous crops, such as peas, beans, 

 vetches, lupins, and the like, the crop contained more nitrogen 

 than could be accounted for from the known sources of supply. 

 Here was a thoroughly established fact that could not be 

 explained by any of the recognised canons of the vegetable 

 physiology of the time. The excess of nitrogen fixed by these 

 leguminous plants was for the time being a mystery that no 

 one could solve. The accuracy of the observations and 

 analyses were in due season confirmed by other inquirers, but 

 no clue to the explanation of the unaccountable excess of 

 nitrogen accumulated in these plants could be so much as 

 suggested. 



The fact that leguminous plants contain more combined 

 nitrogen than could have been drawn from the known sources 

 of supply in the soil and air was at once seen to be in harmony 

 with the experience of agriculture in ancient as well as in 

 modern times. Even the Eomans had believed that legu- 

 minous crops grown on impoverished land greatly improved 

 its fertility, and it was a commonplace of agricultural practice 

 that cereals and grass crops grew much better after a crop of 

 pulse than after any other. 



Inquiry into the source of the excess of nitrogen fixed by 

 leguminous plants was at once taken up, and all sorts of 

 opinions were formed and given to the world. But there w^as 

 no agreement or approach to agreement among inquirers. 

 More than twenty years had to pass by before any real light 



