Fig. 28.—Forms of brown seaweeds. A, Ectocarpus, × 50. Sporangia (sp.). B, a single sporangium, × 150. C, kelp (Laminaria), × ⅛. D, E, gulf weed (Sargassum). D, one-half natural size. E, natural size. v, air bladders. x, conceptacle bearing branches.
The highest of the class are the gulf weeds (Sargassum), plants of the warmer seas, but one species of which is found from Cape Cod southward ([Fig. 28], D, E). These plants possess distinct stems and leaves, and there are stalked air bladders, looking like berries, giving the plant a striking resemblance to the higher land plants.
CHAPTER VII.
Class III.—The Red Algæ (Rhodophyceæ).
These are among the most beautiful and interesting members of the plant kingdom, both on account of their beautiful colors and the exquisitely graceful forms exhibited by many of them. Unfortunately for inland students they are, with few exceptions, confined to salt water, and consequently fresh material is not available. Nevertheless, enough can be done with dried material to get a good idea of their general appearance, and the fruiting plants can be readily preserved in strong alcohol. Specimens, simply dried, may be kept for an indefinite period, and on being placed in water will assume perfectly the appearance of the living plants. Prolonged exposure, however, to the action of fresh water extracts the red pigment that gives them their characteristic color. This pigment is found in the chlorophyll bodies, and usually quite conceals the chlorophyll, which, however, becomes evident so soon as the red pigment is removed.
The red seaweeds differ much in the complexity of the plant body, but all agree in the presence of the red pigment, and, at least in the main, in their reproduction. The simpler ones consist of rows of cells, usually branching like Cladophora; others form cell plates comparable to Ulva ([Fig. 30], C, D); while others, among which is the well-known Irish moss (Chondrus), form plants of considerable size, with pretty well differentiated tissues. In such forms the outer cells are smaller and firmer, constituting a sort of rind; while the inner portions are made up of larger and looser cells, and may be called the pith. Between these extremes are all intermediate forms.
They usually grow attached to rocks, shells, wood, or other plants, such as the kelps and even the larger red seaweeds. They are most abundant in the warmer seas, but still a considerable number may be found in all parts of the ocean, even extending into the Arctic regions.
Fig. 29.—A, a red seaweed (Callithamnion), of the natural size. B, a piece of the same, × 50. t, tetraspores. C i–v, successive stages in the development of the tetraspores, × 150. D I, II young procarps. tr. trichogyne. iii, young; iv, ripe spore fruit. I, III, × 150. iv, × 50. E, an antheridium, × 150. F, spore fruit of Polysiphonia. The spores are here surrounded by a case, × 50.