connected mth the Theory of Agriculture. 385 



Now it is highly important to ascertain the range within 

 which this substitution can go on, because the result may serve 

 to enlighten us as to the extent to which soda, and conse- 

 quently common salt, are capable of supplying the want of 

 potass — as to whether a soluble salt of lime will serve in the 

 place of an alkali — and how far, in short, art has substitutes in 

 store for the ingredients natural to a plant which may chance 

 to be deficient. 



Such an inquiry is the more important, because the views 

 of physiologists on this subject strike one as not altogether 

 consistent either with each other, or with themselves. Liebig 

 for instance, in p. 66 et seq. of his Chemistry in its Applica- 

 tion to Agriculture, seems to admit this power of substitution, 

 and yet his whole theory of rotation, fallow, &c, appears to 

 favour the contrary assumption. It is probable indeed that 

 both principles are true within certain limits, and it therefore 

 remains for future experimentalists to ascertain what these 

 limits may be. 



The inquiry may for convenience sake be distinguished as 

 follows : — 



1. How far have plants the power of substituting one fixed 

 principle for another ? 



2. How far will their growth be affected by the absence of 

 one of the ingredients which they usually contain, provided 

 the rest are supplied to them in a sufficient quantity ? 



3. Are there any ingredients which, if present, are taken up 

 by a plantj but the absence of which does not affect its growth 

 or healthy condition ? 



4. What becomes of a fixed ingredient which a plant does 

 not assimilate? Is it first absorbed and afterwards excreted, 

 or is it rejected in the first instance by any power of selection 

 which the secreting surfaces may possess? 



The next train of inquiries is suggested by the apparent in- 

 consistency between the statements of Liebig in his last work, 

 his Chemical Letters, and those of his former publication, on 

 Chemistry in its Application to Agriculture. 1 say the appa- 

 rent inconsistency, because I think a careful consideration of 

 his remarks may lead us to infer, that what he intended to 

 convey was merely the paramount importance of furnishing a 

 plant with its inorganic constituents, to which the providing 

 it with its organic principles was only subordinate. Possessed 

 of the former, a plant will in ^/w«e obtain from the atmosphere 

 all the carbon and all the nitrogen which it requires ; debarred 

 from the former, the most liberal supply of carbonic acid and 

 of ammonia will be utterly ineffectual. But to infer from 

 thence, that a plant, in Liebig's opinion, derives no advantage 



Phil. Mag. S. 3. Vol. 25. No. 167. Nov. 1844. 2 C 



