[Illustration:  FIG. 2. Ulva lactuca, A GREEN-SEAWEED, GREATLY MAGNIFIED TO SHOW STRUCTURE. (After Orested).

s, Spores in the cells, ss, Spores swimming out. h, Holes through which spores have escaped.]

Let us begin with seaweeds.  I have said that there are three leading colors in my pool—­green, olive, and red—­and these tints mark roughly three kinds of weed, though they occur in an endless variety of shapes.  Here is a piece of the beautiful pale green seaweed, called the Laver or Sea-Lettuce, Ulva Linza (1, Fig. 1),[1] which grows in long ribbons in a sunny nook in the water.  I have placed under the first microscope a piece of this weed which is just sending out young seaweeds in the shape of tiny cells, with lashes very like those we saw coming from the moss-flower, and I have pressed them in the position in which they would naturally leave the plant.  You will also see on this side several cells in which these tiny spores are forming, ready to burst out and swim; for this green weed is merely a collection of cells, like the single-celled plants on land.  Each cell can work as a separate plant; it feeds, grows, and can send out its own young spores.

[Footnote 1:  The slice given in Fig. 2 is from a broader-leaved form, U. lactuca, because this species, being composed of only one layer of cells, is better seen. Ulva Linza is composed of two layers of cells.]

This deep olive-green feathery weed (2, Fig. 1), of which a piece is magnified under the next microscope (2, Fig. 3), is very different.  It is a higher plant, and works harder for its living, using the darker rays of sunlight which penetrate into shady parts of the pool.  So it comes to pass that its cells divide the work.  Those of the feathery threads make the food, while others, growing on short stalks on the shafts of the feather, make and send out the young spores.

Lastly, the lovely red threadlike weeds, such as this Polysiphonia urceolata (3, Fig. 1), carry actual urns on their stems like those of mosses.  In fact, the history of these urns (see 3, Fig. 3), is much the same in the two classes of plants, only that instead of the urn being pushed up on a thin stalk as in the moss, it remains on the seaweed close down to the stem, when it grows out of the plant-egg, and the tiny plant is shut in till the spores are ready to swim out.

[Illustration:  FIG. 3.  THREE SEAWEEDS OF FIG. 1 MUCH MAGNIFIED TO SHOW FRUITS. (Harvey.)

2, Sphacelaria filicina. 3, Polysiphonia urceolata. 4, Corallina officinalis.]

The stony corallines (4, Figs. 1 and 3), which build so much carbonate of lime into their stems, are near relations of the red seaweeds.  There are plenty of them in my pool.  Some of them, of a deep purple color, grow upright in stiff groups about three or four inches high; and others, which form crusts over the stones and weeds, are a pale rose color; but both kinds, when the plant dies, leaving the stony skeleton (1, Fig. 4), are a pure white, and used to be mistaken for corals.  They belong to the same order of plants as the red weeds, which all live in shady nooks in the pools, and are the highest of their race.