The air-bladders of the seaweeds are natural buoys, by means of which the plants are kept erect in the water. The mucilage which makes them so slippery to walk over is of the utmost importance, as it protects them from drought when they are left uncovered by the tide. Seaweeds are very simple in their structure, and have no true roots, stems, or leaves. They do not need such organs, for every part of their body is in contact with the water which contains their food-supply. What are those tufts of reddish-brown threads growing all over the fronds of this Fucus? That is a red seaweed (_Polysiphonia_), which often makes its home under the shelter of a more hardy plant. In the red algæ the sperms have no cilia, and cannot move about of themselves, but the eggs have each a long thread, corresponding to the stigma of the higher plants, and against this thread the sperms are driven by currents of water. The little Fucus known as “teeting tang” (_Fucus canaliculatus_) ought not to be passed unheeded. It is often much relished by sheep and cattle. You may know it by its greenish-brown colour and by the distinct groove on one side all along its length. It is found only in the upper part of the “ebb”. Another interesting plant of this group may be found on the large rocks nearer low-water mark. It is called the “sea-thong” (_Himanthalia lorea_), because its fructification grows out from a button-shaped base into long, forked, thong-like branches. [Illustration: _Common seaweeds.—II._ F, _Fucus canaliculatus_. G, _Himanthalia lorea_. H, _Laminaria digitata_. I, _Rhodymenia_. K, _Chondrus crispus_. L, _Porphyra_.] If the tide is far out, we shall be able to see the tops of the “red-ware” standing out of the water, and some of the tangles will be quite dry. These tangles belong to the _Laminaria_ group, the giants among the seaweeds. They contain a large amount of iodine in their composition, and that is why they are used for the manufacture of kelp. Notice how firmly they cling to the sea-bottom by their strong holdfasts, which have weathered many a storm. An interesting feature in this group is their manner of growth. The growing region lies at the junction of the stalk with the blade. You will often find a specimen in which the old blade is being pushed away on the end of the young one, ready to be broken off and cast adrift by the waves. The stalk itself is perennial, but in some kinds of Laminaria (_Laminaria digitata_, for example) the blade is usually torn into shreds before it is thrown off.