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,