CULTIVATION. 19 



stances thus taken up may either be deposited in the 

 solid state in the cells of the plant, or may remain 

 dissolved in the juices. In tobacco, a part of the nitrogen 

 usually exists as a nitrate, in combination with potash. 

 That is to say, portions of the nitrogenous food of the 

 plant — the nitrates of the soil — are not completely worked 

 over into albuminoids, and into nicotine, the nitrogenous 

 constituents of tobacco, but accumulate and remain in 

 considerable quantity in the sap. When a dry tobacco- 

 leaf is set on fire, it often burns like ' touch paper ' 

 (paper soaked in a solution of saltpetre and dried) with 

 bright sparkles of fire, indicating the points where the 

 nitre has gathered in minute crystals as the juice of the 

 leaf evaporated. The quantity of superfluous salts in the 

 plant depends upon its succulence, and upon the supply 

 of them in the soil. Doubtless certain definite amounts 

 of potash, lime, magnesia, iron, sulphuric acid and phos- 

 phoric acid are absolutely necessary to produce a given 

 weight of tobacco. In case several or all these substances 

 are superabundant in the soil, the plant has no power to 

 exclude any unnecessary surplus of one or all of them 

 from its interior altogether, although there are good 

 reasons known to prevent their entrance beyond a certain 

 limit. In one soil potash may be relatively most abun- 

 dant, and may for that reason be found in the crop in 

 greater quantity than was necessary for the growth of 

 that crop. In another soil lime may be in surplus, and 

 there the crop may have the minimum of potash, and a 

 considerable excess of lime. 



" The crop is a result of the working together of a 

 number of causes or conditions ; these are the heat and 



2 



