A diatom (Diatoma vulgare) growing.
a, b, Flint skeleton inside the jelly-cell. a, c and d, b, Two flint skeletons formed by new valves, c and d, forming within the first skeleton.
The next two forms, a and c, look much more like plants, for the cells arrange themselves on a jelly stem, which by and by disappears, leaving only the separate flint skeletons such as you see in Fig. 16. The last form, d, is something midway between the other forms, the separate cells hang on to each other and also on to a straight jelly stem.
Another species of Diatoma (Fig. 70) called Diatoma vulgare, is a very simple and common form, and will help to explain how these plants grow. The two flinty valves a, b inside the cell are not quite the same size; the older one a is larger than the younger one b and fits over it like the cover of a pill-box. As the plant grows, the cell enlarges and forms two more valves, one c fitting into the cover a, so as to make a complete box ac, and a second, d, back to back with c, fitting into the valve b, and making another complete box bd. This goes on very rapidly, and in this plant each new cell separates as it is formed, and the free diatoms move about quite actively in the water.
If you consider for a moment, you will see that, as the new valves always fit into the old ones, each must be smaller than the last, and so there comes a time when the valves have become too small to go on increasing. Then the plant must begin afresh. So the two halves of the last cell open, and throwing out their flinty skeletons, cover themselves with a thin jelly layer, and form a new cell which grows larger than any of the old ones. These, which are spore-cells, then form flinty valves inside, and the whole thing begins again.
Now though the plants themselves die, or become the food of minute animals, the flinty skeletons are not destroyed, but go on accumulating in the waters of ponds, lakes, rivers, and seas, all over the world. Untold millions have no doubt crumbled to dust and gone back into the waters, but untold millions also have survived. The towns of Berlin in Europe and of Richmond in the United States are actually built upon ground called "infusorial earth," composed almost entirely of valves of these minute diatoms which have accumulated to a thickness of more than eighty feet! Those under Berlin are fresh-water forms, and must have lived in a lake, while those of Richmond belong to salt-water forms. Every inch of the ground under those cities represents thousands and thousands of living plants which flourished in ages long gone by, and were no larger than those you will see presently under the microscope.
These are a very few of the microscopic inhabitants of my pond, but, as you will confuse them if I show you too many, we will conclude with two rather larger specimens, and examine them carefully. The first, called the Cydippe, is a lovely, transparent living ball, which I want to explain to you because it is so wondrously beautiful. The second, the Sea-mat or Flustra, looks like a crumpled drab-coloured seaweed, but is really composed of many thousands of grottos, the homes of tiny sea-animals.
Fig. 71.
Cydippe Pileus.