ON RECENT DEVELOPMENTS IN AGRICULTURAL SCIENCE. 277 
‘evidence that excess of phosphoric acid leads to increased assimilation of 
nitrogen. 
Some of the barley plots at, Rothamsted show this very clearly ; where 
there has been no phosphatic, but a nitrogenous, manuring for the last 
fifty years, the amount of nitrogen assimilated by the crop is diminished, 
but the gross production of dry matter is still further diminished. By the 
addition of phosphoric acid the gross production is increased to a greater 
degree than the amount of proteid formed is increased, so that the crop 
shows now a smaller percentage of nitrogen and a lower ratio of nitrogen 
to phosphoric acid than on the plots which are experiencing phosphoric- 
acid starvation. In other words, where an excess of nitrogen is avail- 
able the amount assimilated does not increase pari passw with the 
amount of phosphoric acid which the plant can obtain. 
But with these three substances all exact knowledge ceases : magnesia, 
sulphuric acid, and chlorine are invariable and necessary constituents of 
all plants, yet their function and their practical effects are still unknown. 
To take a further example, it was early in the history of agricultural science 
that silica was discovered to be the chief constituent of the ash of cereals 
and of a few other plants. Liebig’s term of ‘silica plants’ still survives 
to show the importance once attached to this body, and the earlier experi- 
menters with manures used soluble silicates with the idea of thereb 
increasing the stiffness of straw. But further investigations showed that 
cereals could be brought to maturity without any supply of silica, and 
that the stiffness of the straw was a physiological matter in no way 
conditioned by silica. As a consequence this plant constituent has now 
been disregarded for a long time. But it is idle to suppose that a 
substance present, for example, to the extent of 60 per cent. or so in the ash 
of the straw of wheat, has no part to play in the nutrition of the plant. 
Among the Rothamsted experiments there are fortunately some barley 
plots which have received soluble silica for many years, and a recent 
examination of the material grown on these plots begins to cast some light 
on the function of silica. Its effect upon the plant is in some way parallel 
to that of phosphoric acid ; on the plots which have had no phosphatic 
manure for more than fifty years an addition of soluble silica increases 
the crop, increases the proportion of grain, and hastens the maturity in 
exactly the same fashion as, though toa lesser degree than, an addition of 
phosphoric acid. The results point to the plant rather than the soil as 
being the seat of the action ; a plant that is being starved of phosphoric 
acid can economise and make more use of its restricted portion if a 
quantity of soluble silica be available. There is no possibility of replacing 
phosphoric acid by silica in the general nutrition of the plant, but the 
abundance of silica at the disposal of the cereals certainly enables them to 
diminish their call for phosphoric acid from the soil. 
Much in the same direction lie the researches which are being pursued 
with so much vigour by Loew and his pupils in Japan on the stimulus to 
assimilation and plant development which is brought about by infinitesimal 
traces of many metallic salts not usually recognised as being present 
in: plants at all. It has been often recognised that substances which are 
toxic to the cell in ordinary dilutions may, when the dilution is pushed to 
an extreme, reach a point at which their action is reversed and begins 
to stimulate. Probably some of the materials used as fungicides and 
inhibitors of disease actin this fashion by strengthening the whole con- 
stitution of the plant rather than by directly destroying or checking the 
