402 HOW CROPS GROW. 



salt 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 proportion of 

 the salt, a proportion equal to that in the soil- water, 

 more cannot enter it. So soon as a molecule of the salt 

 has gone on into another cell or been removed from the 

 sap by any chemical transformation, then a molecule 

 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 foli- 

 age, while the leaves and stems of the latter contain but 

 3 parts. 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 itself, brings 

 the cell-contents to the same state of saturation that 

 exists in the soil. The cereals are able to dispose of 

 silica by giving it a place in the cuticular cells ; the 

 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 impos- 

 sible except as room is made by new growth. It is in 

 this way that we have a rational and adequate explana- 

 tion of the selective power of the plant, as manifested 

 in its deportment towards the medium that invests its 

 roots. The same principles govern the transfer of mat- 

 ters from cell to cell, or from organ to organ, within the 

 plant. Wherever there is unlike composition of two 

 miscible juices, diffusion is thereby set up, and proceeds 

 as long as the cause of disturbance lasts, provided im- 

 penetrable membranes do not intervene. The rapid 

 movement of water goes on because there is great loss of 

 this liquid ; the slow motion of silica is a consequence 

 of the little use that arisos for it in the plant. 



Strong chemical affinities may V overcome by help of 



