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.

[Illustration: FIG. 4. CORALLINE AND SERTULARIA, TO SHOW LIKENESS
BETWEEN THE ANIMAL SERTULARIA AND THE PLANT CORALLINE.

1, _Corallina officinalis._ 2, _Sertularia filicula._]

My pool is full of different forms of these four weeds. The green
ribbons float on the surface rooted to the sides of the pool, and, as
the sun shines upon it, the glittering bubbles rising from them show
that they are working up food out of the air in the water, and giving
off oxygen. The brown weeds lie chiefly under the shelves of rocks,
for they can manage with less sunlight, and use the darker rays which
pass by the green weeds; and last of all