But if there are so many thousands of mouths to feed, on the tree-like Sertulariae as well as in all these Infusoria, where does the food come from?  Partly from the numerous atoms of decaying life all around, and the minute eggs of animals and spores of plants; but besides these, the pool is full of minute living plants—­small jelly masses with solid coats of flint which are moulded into most lovely shapes.  Plants formed of jelly and flint!  You will think I am joking, but I am not.  These plants, called Diatoms, which live both in salt and fresh water, are single cells feeding and growing just like those we took from the water-butt, only that instead of a soft covering they build up a flinty skeleton.  They are so small, that many of them must be magnified to fifty times their real size before you can even see them distinctly.  Yet the skeletons of these almost invisible plants are carved and chiselled in the most delicate patterns.  I showed you a group of these in our lecture on magic glasses, and now I have brought a few living ones that we may learn to know them.  The diagram (Fig. 7) shows the chief forms you will see on the different slides.

The first one, Sacillaria paradoxa (b, Fig. 7), looks like a number of rods clinging one to another in a string, but each one of these is a single-celled plant with a jelly cell surrounding the flinty skeleton.  You will see that they move to and fro over each other in the water.

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.  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.

[Illustration:  FIG. 8.  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.]

Another species of Diatoma (Fig. 8) 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 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.