AGRICULTURAL CHEMISTRY. 177 



figure. The liquids of the root-cells being of different composition 

 from the soil-water, and the cell-membranes admitting (having surface 

 attraction for) the soil-water, the latter with its contents penetrates 

 the cells, so long as difference of composition or want of equilibrium in 

 the surface attractions, either of the membrane for the liquid, or of 

 the dissolved matters for the solvent, exist. The diffusion goes on 

 from cell to cell in the same manner throughout the whole plant, as 

 long as any cause produces inequality in the mutual surface attractions 

 of any two of its ingredients, whether solid or liquid. 



Since perpetual changes are progressing in every part of the grow- 

 ing vegetable organism, we have no difficulty in finding the causes 

 which keep up diffusion in or into the plant. 



Let us suppose that in any cell there exists at the moment a liquid 

 containing in solution all the food of vegetation. If now carbonic 

 acid and water unite to form dextrin, and this solidifies in the shape 

 of starch or cellulose, there is formed in this cell a vacuum which 

 disturbs the osmotic equilibrium of the whole plant, and determines 

 a movement towards this cell of carbonic acid from the leaf cells and 

 of water from the root cells to restore the same. 



An atom of lime coming in contact with newly formed oxalic acid 

 combines with it to form an insoluble salt; the lime thus removed 

 from solution is at once replaced from an adjacent cell; this again 

 supplies itself from another in the direction of the soil, until the 

 extremity of a rootlet is reached, and here an atom passes in from 

 the soil water, this again to be replaced from the surrounding stores. 



The vast amount of water that is removed by evaporation (the 

 attraction of dry air for water) from the foliage of vegetation is in 

 the same manner supplied from the soil, and it traverses in its upward 

 way all the cells of the plant. The supply of saline matters is 

 however partially or wholly independent of this ascending current of 

 water, for it must be very greatly checked in circumstances where 

 the atmosphere is saturated with moisture, as in a conservatory or 

 Wardian case, although here growth goes on with the greatest vigor. 



It thus appears that whenever any chemical or physical change 

 occurs in the plant, we have the origin of a disturbance which may 

 set in motion the juices of the cells, the water, and dissolved matters 

 of the soil, and the gases of the atmosphere. 



In this manner our cultivated plants are able to gather their food 

 from solutions like the water of springs and wells, or the aqueous 

 extract of soils, which are so dilute that but one part of potash or 

 phosphoric acid is present in one or even twenty thousand parts of 

 water. So, too, we may find in plants, substances which it is im- 

 possible to detect in the soil, and it is not a little interesting that 

 iodine, a substance largely employed in medicine and photography, 

 is almost entirely procured from the ashes of sea-weeds, although it 

 has never yet been detected with certainty in sea-water, even by 

 the use of methods that would enable the chemist to find it, did it 

 form but one part in a million. 



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