CAUSES OP THE MOTION OP JUICES. 363 



Other by penetrable membranes. This tendency makes 

 valid for the organism of the plant the law that demand 

 creates supply. In two contiguous cells, one of which 

 contains solution of sugar, and the other, solution of ni- 

 trate of potash, these substances must diffuse until they 

 are mingled equally, unless, indeed, the membranes or some 

 other substance present exerts an opposing and preponder- 

 ating attraction. 



In the simplest phases of diffusion each substance is to 

 a certain degree independent of every other. Nitrate of 

 potash dissolved in the water of the soil must diffuse into 

 the root-cells of a plant if it be absent from the sap of this 

 root-cell and the membrane permit its passage. When 

 the root-cell has acquired a certain i^roportion of nitrate 

 of potash, a proportion equal to that in the soil-water, the 

 nitrate cannot enter it any more. So soon as a molecule 

 of the salt has gone on into another ceil or been removed 

 from the sap by any chemical transformation, then a mold- 

 cule may and must enter from without. 



Silica is much more abundant in grasses and cereals than 

 in leguminous plants. In the former it exists to the extent 

 of about 25 parts in 1,000 of the air-dry foliage, while the 

 leaves and stems of the latter contain but 3 parts. (See 

 Wolff's Table in Appendix.) When these crops grow side 

 by side, their roots are equally bathed by the same soil- 

 water. Silica enters both alike, and, so far as regards it- 

 self, brings the cell-contents to the snme state of satura- 

 tion that exists in the soiH The cereals are able to dispose 

 of silica by giving it a place in the cutioular cells ; tbe 

 leguminous crops, on the other hand, cannot remove it 

 from their juices ; the latter remain saturated, and thus 

 further diffusion of silica from without becomes impossi- 

 ble except as room is made by new growth. It is in this 

 way that we have a rational and adequate explanation of 

 the selective power of the plant, as manifested in its de- 

 portment towards the medium that invests its roots. The 

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