364 HOW CROPS GROW. 



same principles govern the transfer of matters from cell 

 to cell, or from organ to organ, within the plant. Where- 

 ever there is unlike composition of two miscible juices, 

 diffusion is thereby set up, and proceeds as long as the 

 cause of disturbance lasts, provided impenetrable mem 

 branes 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 arises 

 for it in the plant. 



Strong chemical affinities may be overcome by osmose. 

 Graham long ago observed the decomposition of alum 

 (sulphate of alumina and potash,) by mere diffusion ; its 

 sulphate of potash having a higher diffusive rate than its 

 sulphate of alumina. In the same manner acid sulphate 

 of potash, put in contact with water, separates into sul 

 phate of potash and free sulphuric acid. 



We have seen (pp. 170-1) that the plant when vegetat 

 ing in solutions of salts, is able to decompose them. It 

 separates the components of nitrate of potash appropriat 

 ing the acid and leaving the base to accumulate in the 

 liquid. It resolves chloride of ammonium, taking up am 

 monia and rejecting the chlorine. The action in these 

 cases, we cannot definitely explain, but our analogies 

 leave no doubt as to the general nature of the agencies 

 that cooperate to such results. 



The albuminoids in their usual form are colloid bodies 

 and very slow of diffusion through liquids. They pass a 

 membrane of nitrocellulose somewhat (Schumacher) ; but 

 can scarcely penetrate parchment-paper. (Graham.) In 

 the plant they are found chiefly in the sieve-cells and ad 

 joining parts of the cambium. Since for their production, 

 they undoubtedly require the concourse of a carbohydrate 

 and a nitrate, they are not unlikely genera U d in the cam 

 bium itself, for here the descending carbohydrates from 

 the foliage ccnie in contact \vith the nitrates as they rise 

 from the soiL On the other hand, the albuminoids be 



