Closely related to Vaucheria, but not quite so common, is the curious little plant known as Botrydium Granulatum. Like our previous example, it is a one-celled plant and herein lies its interest. When find a specimen growing by a pond, we shall notice a green bladderlike portion, not more than 1/6 inch in diameter, which projects above the ground. Below ground, if we pick the soil away carefully we may observe a number of colourless, branched structures which do duty for roots. The bladder and underground portions are hollow, being lined in the case of the former, with a network of chlorophyll beautiful to observe under the microscope.
Botrydium usually increases by buds which form on the green bladder, break away and grow into new plants. Should the level of the water in the pond rise, and the little plant become submerged, it splits up into a number of small bodies, each provided with a minute whip-like organ, with which they swim rapidly ashore where they develop into new plants. In dry weather the plant forms a number of little cells each one surrounded by a firm cell wall. When moisture comes again these little cells give rise to structures which will grow into new plants.
Surpassing even Drapernalda Glomerata in point of beauty, is the “water-net,” Hydrodictyon Reticulatum, but it is not nearly so common, being confined to ponds in the South and Midlands. When full grown it hardly comes under the heading of a microscopic object, for it may measure as much as six inches in length. This remarkable pond plant consists of an open network of green filaments. Apart from its striking appearance the most remarkable thing about the “water net” is its rapid growth. Carpenter, a noted microscopist says:—“The original cells of which the net is composed measure one-two thousand five hundredth part of an inch in length but in a few hours they grow to one-twelfth of an inch or 1/3 of an inch in length. We often hear people remark that they can see plants grow, but their statements are not literally true; in the case of the ‘water net’ it is actually possible to see the growth.”
By the side of our pond we shall probably observe some masses of a bluish-green jelly-like substance. It is uninteresting-looking material for the microscope, but we must not pass it by. The blue-green jelly encloses a plant called Nostoc, which resembles nothing so much as a necklace of beads; this we can plainly see under a low magnification. We shall observe that the plant is twisted spirally within its covering and also that most of the cells, which we have compared to beads, are similar to one another in size. At intervals there are larger cells and as the plant increases in length they are emptied periodically, so they are evidently used as food stores. From time to time portions of the plant break away, worm their way out of the jelly and move about, rather after the manner of a worm. Eventually, these wanderers come to rest and become surrounded with more of the blue-green jelly, thus forming a new Nostoc colony.
These plants sometimes appear, apparently from nowhere, on garden paths, walls and similar situations, during damp autumn weather. On this account the plants have been called “fallen stars.” Their appearance is not so mysterious as it might seem for the Nostoc colony has probably been where it is found, all through the summer, in a dried up, contracted state. Only when rain comes, does the jelly envelope absorb water, swell up and assume its normal appearance. The blue-green scum which floats on stagnant water is a closely related plant.
Another plant which looks like scum on the water is known as Spirogyra and a very beautiful object it makes for the microscope. It is bright green, without a bluish tinge, so it need not be confused with the plant we have just mentioned. There are seventy or so different kinds of Spirogyra, therefore our description must be the one that will apply to all. The plant is thread-like and, even in the larger kinds, the threads are not more than one-hundredth part of an inch in diameter. Like Oscillatoria the plants are not attached to any support. Each thread is composed of many cells, arranged end to end; we can distinguish the cell walls clearly, but what will chiefly attract our attention are the beautiful bands of green colouring matter, running spirally, round each cell. If we are fortunate, we shall see the plant in the act of increasing; this is not the simple operation we witnessed in the silk weeds. From two threads lying parallel to one another, we shall see swellings arise on the adjacent cell walls; the beautiful spiral bands will begin to break up at the same time and to collect in a mass towards the centre of each cell. The swellings of adjacent cells touch, their end walls break down so that the two cells become connected, our figure shows the fusion taking place, then the contents of one cell passes into the adjacent cell and fuses with the contents of the latter. The new cell, which now contains not only its original contents but that from a cell in another plant, becomes detached from the thread, assumes an oval shape and sinks to the bottom of the pond, where it rests awhile. At a later period the cell wall bursts and a new thread of Spirogyra develops from it.
When we are examining Spirogyra we may notice some very minute brick-red, spherical bodies adhering to the green threads. These are little pond animals, known by the name of Vampyrella Spirogyræ. This little creature passes through an interesting and easily observed series of changes. Its life is very uneventful, consisting of a good meal of Spirogyra, a period of rest followed by an increase, and this is repeated over and over again. If we watch our little sphere through the microscope, we may be lucky enough to see the contents divide into four parts. Now we must watch carefully, for very interesting events are about to take place. Each of the four parts into which the contents of the sphere has split, escapes into the water and swims about for a time; it then becomes spherical, but instead of having a smooth outer surface, as it had when we first observed it, we can see that it is now studded all over with very fine threads. It then wanders along a Spirogyra plant, attaches itself to one of the cells, perforates its wall and sucks out the contents. This performance it repeats several times then, evidently satiated, it loses its threads and resumes the appearance it had when first observed and thus it rests, till division of its contents into four parts takes place once more and the little comedy is repeated.
Now we must tear ourselves away from our pond; there are very many interesting objects for our microscope on the sea shore and if we delay too long here, we shall not be able to give them the attention they deserve.