All the Fucaceæ are tough leathery plants. This is even characteristic of the genus Cystoseira, various species of which may be seen on our coasts at low water mark, or in the tide pools. They are little shrub-like and somewhat thorny plants, not more than three feet high, with a cylindrical stem and many branches, near the extremities of which there are inflated air-vessels, sometimes two or three together; in some species they are lower down. Long spiny conceptacles are situated at the tips of the branches, but the endochrome does not divide in the germ cells as it does in the Fuci, so that each cell produces but one spore. ‘Throughout all latitudes the two divisions of Fucaceæ—Fucoideæ and Cystoseireæ, form the prevailing marine vegetation to which the name of sea-weed is commonly applied, and the different genera so arrange themselves as to present, with a few exceptions, a most harmonious assemblage.’ ‘None of these approach the tropics; the Fucoideæ abound towards the poles, and there attain their greatest bulk, diminishing rapidly towards the equator, and ceasing some degrees from the line itself; while the immense genus Sargassum finds its maximum in lower latitudes and under the equator itself. In the opposite cold and frigid zones the waters are inhabited by certain genera of Fucoideæ, which are in a great measure representatives of one another.’[50] The huge D’Urvillæa and the Sarcophycus in the Antarctic Ocean represent the Himanthalia and Fucus proper in the north, and the Cystoseireæ and Halidrys of the northern seas are represented by the Blossevillea and Scytothalia in the southern.