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 codperate 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 generated in the cam- 
bium itself, for here the descending carbohydrates from 
the foliage come in contact with the nitrates as they rise 
from the soil On the other hand, the albuminoids be 
