Now let us turn our attention to the seaweeds which we find growing on the beach around us. In many a rock pool in the “ebb” we may see a miniature forest of tiny weeds of beautiful colours and forms, a veritable ocean garden. Near high-water mark we find here and there in the pools pretty green algæ, some with broad, flat fronds, such as the sea-lettuce (_Ulva_), and others with slender branching feathery filaments (_Cladophora_). Many of the green algæ, however, prefer to live in fresh water. If you make an aquarium, you will find the sea-lettuce and the sea-grass (_Enteromorpha_) of great value in keeping the water pure, owing to the amount of oxygen which they give out. Farther down on the beach the rocks are covered thickly with algæ of an olive-brown colour. The rocks, indeed, would fare much worse in a storm if the seaweeds were not there to protect them, as the grass protects the soil of the fields. Look more closely at those big brown sea-wracks and you will notice that the most common kind (_Fucus vesiculosus_) has little globular air-bladders arranged in pairs along its flat, smooth-edged fronds. Each blade has a distinct midrib, and where it divides, like all the Fucus group, it splits into two equal branches. On some of the little end branches you may see a yellowish swelling dotted over with minute knobs and pores. These swellings are receptacles for holding the eggs and sperms, which are contained in tiny cavities under each projecting knob. Many seaweeds produce their fruit in winter, when the land plants are sleeping and the fields are bare. The microscopic sperms correspond to the pollen and the eggs to the ovules of the flowering plants. But there is one wonderful difference. The sperms of the Fucus can move about freely by means of two little projecting threads or cilia. When the tide is out, both eggs and sperms come to the door of their little houses by the help of the mucilage in which they float; and when the sea comes back swarms of these sperms swim away and wriggle about, till one of them comes in contact with an egg. It adheres to and fuses with the egg, which thus becomes fertilized, and is then able to give rise to a young plant. A similar process goes on in all the plants of the Fucus group. Here is one with notched or serrated edges (_Fucus serratus_), and without air-bladders; there another well known to every schoolboy as the “bell tang” (_Fucus nodosus_), with large air-bladders in the centre line of the frond, and yellow fruit-bodies each on a branch of its own, without any trace of midrib.