. SOME SUBSTANCES ACT AS MEDIA OR AGENTS ONLY. 227 



2°. If the substances necessary for the perfection of one or more parts 

 of a plant abound in the soil, its chief developement will take the direc- 

 tion of those parts. Thus one plant will run to leaf or straw, another to 

 flower and seed. Thus also in the grain of one crop of wheat more glu- 

 ten is produced «han in that of another, and as this gluten appears to 

 contain the phosphates of lime and magnesia, as essential constituents, 

 the ash will necessarily vary with the gluten of the seed. 



3°. Some substances ap{)ear to enter into the circulation of plants not 

 so much as actual and necessary ccnstituents of the parts of the vegetable, 

 as to serve as media or agents by which other compounds, both organic 

 and inorganic, may be conveyed to the plant. Thus common salt ap- 

 pears to enter many plants for the purpose of supplying soda, its chlo- 

 rine being discharged by the leaf. Silica enters the plant chiefly in the 

 form of silicate of potash or soda. When it reaches its proper destina- 

 tion — the stalks of the grasses for instance — this silicate is decomposed 

 chiefly by the carbonic acid, which is always present in the pore^of the 

 green stem, the silica is deposited and the alkali proceeds downwards 

 with the sap as a soluble carbonate, or in combination with some other 

 organic acid. Thus the same portion of alkali may return many times 

 into the circulation with this or with other materials which the parts of 

 the plant require, and every new burden it deposits will necessarily 

 cause a new variation in the relative proportions of the several inorganic 

 constituents which are afterwards detected in the ash. 



4°. As the water which enters by the roots always brings with it some 

 soluble substances, the quantity of these conveyed into the plant will be 

 materially affected by the amount of evaporation from the leaves; and 

 hence, after a long drought, the leaves of the turnip, the potato, and 

 other plants, will yield a larger proportion of ash than will be obtained 

 from them in moist and rainy weather. 



5°. In the mineral kingdom it is found that one substance may not 

 unfrecjuently take the place, and perform the functions, of another. Thus 

 potash and soda replace each other in certain minerals, as do also lime 

 and magnesia and the phosphoric and arsenic acids. It has been sup- 

 posed that a similar interchange may take place in the vegetable king- 

 dom — that when the plant cannot get potash it will take soda — that 

 when it can get neither, it will appropriate lime, — and so on. Such a 

 conjectural interchange may possibly take place in a small degree, for a 

 limited time, and in certain plants, without materially affecting their ap- 

 parent heahh — but it is not by trusting to such resources of nature that 

 a luxuriant vegetation or plentiful crops will ever be reared by the prac- 

 tical agriculturist. 



Admitting, however, all these sources of variation in the kind and 

 quantity of the ash obtained from different plants, the sound practical 

 conclusions from all we know on the subject at present seem to be — 



1°. That certain inorganic substances, in certain proportions, are ne- 

 cessary to all plants usually cultivated for food — if they are to be reared 

 or maintained in a healthy state. 



stalk and the potato require more potash while in rapid growth. This growth may be con. 

 tinued and prolonged by the presence of ammonia ; while lime is said to bring it sooner to 

 a close, and to give an earlr.jr harvesst. How valuable would be the multiplication of such 

 facts! 



