J. II. Gilbert— Points in connection ivith Vegetation. 103 



a more extended period of the season of active growth, than 

 do the gramineous corn crops. 



It may safely be asserted, then, that neither direct experi- 

 mental evidence, nor a consideration of the chemistry and the 

 physics ol the subject, would lead to the conclusion that the 

 plant- which assimilate more nitrogen over a given area than 

 others, do so by virtue of a greater power of absorbing by 

 their leaves combined nitrogen from the atmosphere in -the 

 form of ammonia. And here it may be said in passing that 

 the argument would be still stronger against the supposition 

 that nitric acid in the atmosphere supplies directly to the 

 leaves of plants any important amount of the nitrogen they 

 a« I it 



But apart from the more purely scientific considerations 

 bearing upon the question, we believe that our statistics of 

 nitrogen- production are themselves sufficient to justify the con- 

 clusion that, at any rate, the "broad-leaved" root-crops, turnips 

 and the like, to which the function has with the most confidence 

 been attributed, do not take up any important proportion of 

 their nitrogen by their leaves from combined nitrogen in the 

 atmosphere. Thus, it has already been shown, that the yield 

 of nitrogen in these crops, even with the aid of complex min- 

 eral manures, was in the later years reduced to a lower point 

 than that in any other crop ; the percentage of nitrogen in the 

 upper layers of the soil was also reduced to a lower point than 

 with any other crop. The evidence of this kind is, however, 

 admittedly not so conclusive in regard especially to plants of 

 the leguminous family. 



Is the free nitrogen of the atmosphere the source of the a 

 nitrogen?— But as about four-fifths of the atn osph. r< . hi< h sur- 

 rounds the leaves of plants consist of free nitrogen, w! 

 not this be a source to them of the nitrogen they require? To 

 assume that it is so is such an obvious and easy way out of so 

 many difficulties, that this assumption has from time to time 

 been freely made, and much experimental inv, -: 

 been undertaken on the point with the most conflicting results. 

 It is now nearly forty years since Boussingault showed that 

 there was a greater assimilation of nitrogen over a given area 

 in a rotation of crops than he could well account for; and 

 almost from that time to this he has been occupied with inves- 

 tigations of very various kinds, sometimes on the atmosphere, 

 sometimes on meteoric waters, sometimes on plants, and some- 

 times on soils, the main object of which has obviously been to 

 throw light on the question of the sources of the nitrogen of 

 vegetation. And almost for as long a period as Bon 

 Mr. Lawes and myself have devoted much thought and mves- 



