224 THE OPEN BOOK OF NATURE 
box and its lid. A slimy secretion covers the out- 
side of the cell-wall, which I have likened to a shell, 
and this enables the Diatom to slip about so easily. 
There are many species of Diatoms, each one having 
peculiar markings on its flinty cell-walls. One 
might imagine these markings to be the work of a 
skilful engraver. Diatoms are further examples of 
Nature’s armoured cruisers. We have talked of 
armoured fish, and other armoured animals ; now 
we find examples of armoured plants !*’ Nature 
never ceases to surprise us. 7 
’ These little plants multiply by division, each one 
becoming two. They are interesting to geologists 
as well as botanists, for their flinty skeletons fall to 
the bottoms of lakes and oceans, and form deposits 
known as “ Diatom-earth.” A bed of such material 
occurs at Richmond, Virginia; it is no less than 
thirty feet thick ; so you can imagine the immense 
period of time it would take for such a deposit to be 
made. Diatom-earth is also found under peat-bogs 
in some parts of Britain. 
I must mention another plant that propels itself 
through water, and makes a most interesting object 
for microscopical study : this is the Volvox, which is 
often found in great numbers in clear ponds. This 
organism is scarcely more than a fiftieth of an inch 
in diameter. Itis a tiny globe of green membranous 
matter, quite transparent, enclosing a few cells like 
itself, though, of course, smaller. The little sphere 
is covered by groups of cilia, and each cell contained 
within it has a similar covering. The parent sphere 
propels itself through the water in a revolving 
motion, and the enclosed cells revolve inside the 
