1890-91.] Dr Murray and Mr Irvine on Silica in Seas. 
247 
Here we find silicic acid always present, and in many cases bulk- 
ing very largely of the whole amount of the ash left on incinerating 
the plants. 
On looking at the analyses of such soils, we find a very large 
proportion of their constituents to consist of silicic acid, but in the 
insoluble form of sand, or in chemical combination with alumina as 
insoluble clay. 
In the drainage water from good arable land, the amount of silica 
found amounts only to 1 part in 100,000. There must therefore 
be processes at work by which a plant can render soluble and make 
available the silicic acid of the soil, from which its roots obtain this 
in common with other mineral food. It has frequently been pointed 
out * that when polished slabs of marble, dolomite, or apatite were 
buried in pure sand, in which seeds were planted, wherever the 
roots of the plants reached the slabs corrosion of the surface took 
place, which is explained by stating that the fine rootlets secrete 
acids having a solvent and disintegrating action on the lime-bearing 
rocks. The rootlets have in all probability a similar effect on siliceous 
rocks. In the case of what we consider eminently siliceous plants, 
the silicic acid absorbed from the soil and secreted on the outer cell- 
walls of the stems and in the joints must, it appears to us, have been 
so obtained. The clay, or even the sand grains, has been no doubt 
rendered soluble by plant action, for we have seen that no ordinary 
soil contains soluble silicic acid in the least degree equal to what is 
required for the healthy life of such plants as produce siliceous 
coatings. So far as we know, some plants usually containing silicic 
acid can be grown to vigorous and complete development under 
conditions in which they are entirely deprived of silicic acid ; and 
granting that silicic acid does not appear to be necessary for their 
nutrition, yet we find it present in most plants, just in the same way as 
carbonate of lime, although not necessary for the nutrition of animals 
and plants, yet is always present in Foraminifera, Algae, &c., f where 
we find it always associated as part of the body structure. 
At one time agriculturists supposed that by adding soluble sili- 
cates to the soil, the stems of cereals would be so strengthened that 
* Sachs’ Physiology of Plants, pp. 262, 263. 
t See Pouch et and Chabry, “L’eau de mer artificielle comme agent terato- 
genique,” Journ. de I’Anatomie, 1889, pp. 289-307. 
