TRANSACTIONS OF SECTION B. 521 



chiefly of avail in protecting the subjacent cells and their contents from those rays 

 of light which would he adverse to the secondary processes which have been dis- 

 tinguished as growth ? 



Appropriate as it would seem that I should attempt to lay before you a resume 

 of results bearing upon the points herein involved, so numerous and so varied have- 

 been the investigations which have been undertaken on the several branches of the 

 question in recent years, that adequately to discuss them would occupy the whole 

 time and space at my disposal. I must therefore be content thus to direct atten- 

 tion to the subject and pass on to other points. 



It has been shown that the plant may receive abundance of nitrogen, may pro- 

 duce abundance of chlorophyll, and may be subject to the intiuence of sufficient 

 light, and yet not assimilate a due amount of carbon. On the other hand, it has 

 been seen that the mineral constituents may be liberally provided, and yet, in the 

 absence of a sufficient supply of nitrogen in an available condition, the deficiency in 

 the assimilation of carbon will be still greater. In fact, assuming all the other 

 necessary conditions to be provided, it was seen that the amount of carbon assimi- 

 lated depended on the available supply of nitrogen. 



In a certain general sense it may l)e said that the success of the cultivator may 

 be measured by the amount of carbon he succeeds in accumulating in his crops. 

 And as, other conditions being provided, the amount of carbon assimilated depends 

 on the supply of nitrogen in an available form within the reach of the plants, it is 

 obvious that the question of the sources of the nitrogen of vegetation is one of first 

 importance. 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 ? 



These are questions which Mr. Lawes and myself have discussed so frequently 

 that it might seem some apology was due for recurring to the subject here, espe- 

 cially as 1 referred to it in some of its aspects before this Section at the Sheffield 

 Meeting last year. But the subject still remains one of first importance to agri- 

 culture, and it could not be omitted from consideration in such a review as I have 

 undertaken to give. Moreover, there are some points connected with it still im- 

 settled, and some still disputed. 



It will be remembered that Pe Saussure's conclusion was that plants did not 

 assimilate the free or uncombined nitrogen of the atmosphere, and that the)' 

 derived their nitrogen from the compounds of it existing in the atmosphere, and 

 especially in the soil. Liebig, too, concluded that plants do not assimilate nitrogen 

 from the store of it existing in the free or uncombined state, but that ammonia 

 was their main source, and he assumed the amount of it annually coming down in 

 rain to be much more than we now know to be the case. 



Referring to our previous papers for full details respecting most of the points 

 in question, I will state, as briefly as I can, the main facts known — first in regard 

 to the amount of the measurable, or as yet measured, annual deposition of combined 

 nitrogen from the atmosphere ; and secondly as to the amount of nitrogen annually 

 assimilated over a given area by different crops — so that some judgment may 

 be formed as to whether the measured atmospheric sources are sufficient for the 

 requirements of agricultural production, or whether, or where, we must look for 

 other supplies ? 



First, as to the amount of combined nitrogen coming down as ammonia and 

 nitric acid in the measured aqueous deposits from the atmosphere. 



Judging from the results of determinations made many years ago, partly by 

 Mr. Way, and partly by ourselves, in the rain, &c., collected at Rothamsted ; from 

 the results of numerous determinations made much more recently by Professor 

 Prankland in the deposits collected at Rothamsted, and also in rain collected 

 elsewhere; from the results obtained by Boussingault in Alsace; from those of 

 MaritS-Davy at the Meteorological Observatory at Montsouris, Paris ; and from: 

 those of many others made in France and Germany — we concluded, some years 

 ago, that the amount of combined nitrogen annually so coming down from the 

 atmosphere would not exceed 8 or 10 lbs. per acre per annum in the open country 

 in Western Europe. Subsequent records would lead to the conclusion that this 



