246 Proceedings of Royal Society of Edinburgh. [sess. 
careful conditions of our experiments , sufficient silica for vigorous 
growth. On placing a small portion of the matted sludgy matter 
under the microscope, it was interesting to notice that all round the 
outside, and even piercing into the very centre of the mineral 
matter composing the mud, there were living Diatoms in great 
abundance, whilst in the clearer field of the microscope free from 
clay, only a very few were detected floating about. These experi- 
ments seem to point to the conclusion that these organisms are in 
the process of growth able to obtain their silica from (otherwise) 
insoluble compounds of silicate of alumina. 
(D) A patch of (A) was introduced (August 1890) into culture 
solution containing silicate of lime. The result here was an 
abundant growth of Diatoms. 
(E) A patch of (A) was introduced (August 1890) into culture 
solution containing pure amorphous silica. A very few seemed to 
have lived, hut the major portion had died by December 15, 1890. 
(F) A patch of (A) was introduced (August 1890) into culture 
solution containing Diatom Ooze, and when examined shortly after- 
wards there seemed to be no growth, but subsequently (December 
15) a considerable mass of living Diatoms was observed. This 
may be due to the soluble silica present in Diatom Ooze, but 
of course such a source of silicic acid for surface Diatoms is out 
of the question. However, Sponges may obtain their silicic acid in 
part in this manner. 
Take now the case of land plants growing in a virgin soil, con- 
sisting of decomposing rocks, sand, clay, and salts of lime, potash, 
and so on, or in a barren soil, from which, by repeated cropping, all 
the soluble food salts have been extracted, but to which manure is 
added to replace the salts represented in the culture solution used 
in our experiments. In either case we have bulky crops grown, and 
on examining the ash left on burning the grain or straw we find 
large quantities of silicic acid which has been absorbed, as is shown 
by the following Table : — 
Table IY. 
The ash of wheat straw contains 73 ‘57 per cent, silicic acid. 
33 
barley „ 
„ 32*73 
3 3 
33 
33 
oat ,, 
„ 38-48 
33 
33 
33 
hay 
„ 53-43 
33 
33 
