[Illustration: Fig. 553. Agarum Turneri, Sea Colander (so called from the perforations with which the frond, as it grows, becomes riddled); very much reduced in size.] [Illustration: Fig. 554. Upper end of a Rockweed, Fucus vesiculosus, reduced half or more, _b_, the fructification.] 508. The common Rockweed (Fucus vesiculosus, Fig. 554, abounding between high and low water mark on the coast), the rarer Sea Colander (Agarum Turneri, Fig. 553), and Laminaria, of which the larger forms are called Devil's Aprons, are good representatives of the olive green or brownish Seaweeds. They are attached either by a disk-like base or by root-like holdfasts to the rocks or stones on which they grow. [Illustration: Fig. 555. Magnified section through a fertile conceptacle of Rockweed, showing the large spores in the midst of threads of cells. 556. Similar section of a sterile conceptacle, containing slender antheridia. From Farlow's "Marine Algæ of New England."] 509. The hollow and inflated places in the Fucus vesiculosus or Rockweed (Fig. 554) are air-bladders for buoyancy. The fructification forms in the substance of the tips of the frond: the rough dots mark the places where the conceptacles open. The spores and the fertilizing cells are in different plants. Sections of the two kinds of conceptacles are given in Fig. 555 and 556. The contents of the conceptacles are discharged through a small orifice which in each figure is at the margin of the page. The large spores are formed eight together in a mother-cell. The minute motile filaments of the antheridia fertilize the large spores after injection into the water: and then the latter promptly acquire a cell-wall and germinate. 510. The Florideæ or Rose-red series of marine Algæ (which, however, are sometimes green or brownish) are the most attractive to amateurs. The delicate Porphyra or Laver is in some countries eaten as a delicacy, and the cartilaginous Chondrus crispus has been largely used for jelly. Besides their conceptacles, which contain true spores (Fig. 560), they mostly have a fructification in _Tetraspores_, that is, of spores originating in fours (Fig. 559). [Illustration: Fig. 557. Small plant of Chondrus crispus, or Carrageen Moss, reduced in size, in fruit; the spots represent the fructification, consisting of numerous tetraspores in bunches in the substance of the plant. 558. Section through the thickness of one of the lobes, magnified, passing through two of the imbedded fruit-clusters. 559. Two of its tetraspores (spores in fours), highly magnified.] [Illustration: Fig. 560. Section through a conceptacle of Delesseria Leprieurei, much magnified, showing the spores, which are single specialized cells, two or three in a row.]