vith Vegetation. 21 



words, there is very little variation in the composition of the 

 ash of one and the same description of seed, or other final pro- 

 duct, provided it be evenly and perfectly matured. This fact 

 alone, independently of all that has been established of late 



growth; and it is obvious that they must be provided unthw 

 the soil. 



But now as to the combustible constituents — the carbon, the 

 hydrogen, the oxygen, and the nitrogen. Leaving out of con- 

 sideration such exceptional eases as those brought to light in 

 Mr. Darwin's beautiful investigation on insectivorous plants, 

 and also the sources of the organic substance of fungi, and. per- 

 haps of some forced horticultural productions, it may be slated, 

 that the source of the carbon of vegetation generally ; - the 

 carbonic acid existing in very small proportion, but in large 

 actual amount, in the atmosphere ; that the source of the 

 hydrogen is water; and that the source of the oxygen may be 

 either that in carbonic acid, or that in water. With regard to 

 the nitrogen the case is, however, by no means so simple. Not 

 that there are no questions still open for investigation in regard 

 to the assimilation by plants uf their incombustible or mineral 

 constituents, or of their carbon, their hydrogen, and their oxy- 

 gen ; but those relating to the sources, and to the assimilation, 

 of their nitrogon, are not only in many respects of more impor- 

 tance, but seem to involve greater difficulties in their solution. 



What, then, are the sources of the nitrogen of vegetation ? 

 Are they the same for all descriptions of plants? Are they to 

 be sought entirely in the soil? or entirely in the atmosphere ? 

 or partly in the one, and partly in the other? 



Amount of vi' I, ■!•[!'■ 1 1 curried doicn hi/ iilniox/iheric iirecipitnli'ji). — 

 As the combined nitrogen coming down from the atmosphere 

 in rain, hail, snow, mists, fog, and dew, does undoubtedly con- 

 tribute to the annual yield of nitrogen in our crops, let us first 

 briefly consider what is known as t<> the amount of it annually 

 so coming down over a given area of the earth's surface: and 

 as we are here discussing the subject in England, I will adopt 



as the unit of area. The following table shows the amount of 



nitrogen coming down as ammonia and nitiic acid in the total 

 rain, hail, snow" and some of the minor deposits, during the 

 y n- i<>:\. InV,. and ! -OK at Rothamsted (Herts), the nitric 

 acid being, in all cases determined by Mr. Way, and the 

 in some cases by him, and in others by ourselves: — 



