Elements of Structural and Systematic Botany. Douglas Houghton Campbell
sexual organs of the rock weed, as we have already seen, are borne in special cavities (conceptacles) in the enlarged ends of some of the branches. In the species here figured, F. vesiculosus, the antheridia and oögonia are borne on separate plants; but in others, e.g. F. platycarpus, they are both in the same conceptacle.
The walls of the conceptacle (Fig. 26, B) are composed of closely interwoven filaments, from which grow inward numerous hairs, filling up the space within, and often extending out through the opening at the top.
The reproductive bodies arise from the base of these hairs. The oögonia (Fig. 26, C, E) arise as nearly colorless cells, that early become divided into two cells, a short basal cell or stalk and a larger terminal one, the oögonium proper. The latter enlarges rapidly, and its contents divide into eight parts. The division is at first indicated by a division of the central portion, which includes the nucleus, and is colored brown, into two, four, and finally eight parts, after which walls are formed between these. The brown color spreads until the whole oögonium is of a nearly uniform olive-brown tint.
When ripe, the upper part of the oögonium dissolves, allowing the eight cells, still enclosed in a delicate membrane, to escape (Fig. 27, H). Finally, the walls separating the inner cells of the oögonium become also absorbed, as well as the surrounding membrane, and the eight egg cells escape into the water (Fig. 27, I) as naked balls of protoplasm, in which a central nucleus may be dimly seen.
The antheridia (Fig. 26, F, G) are small oblong cells, at first colorless, but when ripe containing numerous glistening, reddish brown dots, each of which is part of a spermatozoid. When ripe, the contents of the antheridium are forced out into the water (G), leaving the empty outer wall behind, but still surrounded by a thin membrane. After a few minutes this membrane is dissolved, and the spermatozoids are set free. These (Fig. 27, K) are oval in form, with two long cilia attached to the side where the brown speck, seen while still within the antheridium, is conspicuous.
The act of fertilization may be easily observed by laying fresh antheridia into a drop of water containing recently discharged egg cells. To obtain these, all that is necessary is to allow freshly gathered plants to remain in the air until they are somewhat dry, when the ripe sexual cells will be discharged from the openings of the conceptacles, exuding as little drops, those with antheridia being orange-yellow; the masses of oögonia, olive. Within a few minutes after putting the oögonia into water, the egg cells may be seen to escape into the water, when some of the antheridia may be added. The spermatozoids will be quickly discharged, and collect immediately in great numbers about the egg cells, to which they apply themselves closely, often setting them in rotation by the movements of their cilia, and presenting a most extraordinary spectacle (J). Owing to the small size of the spermatozoids, and the opacity of the eggs, it is impossible to see whether more than one spermatozoid penetrates it; but from what is known in other cases it is not likely. The egg now secretes a wall about itself, and within a short time begins to grow. It becomes pear-shaped, the narrow portion becoming attached to the parent plant or to some other object by means of rootlets, and the upper part grows into the body of the young plant (Fig. 27, M).
Fig. 27.—H, the eight egg cells still surrounded by the inner membrane of the oögonium. I, the egg cells escaping into the water. J, a single egg cell surrounded by spermatozoids. K, mass of spermatozoids surrounded by the inner membrane of the antheridium. L, spermatozoids. M, young plant. r, the roots. K, × 300; L, × 600; the others, × 150.
The simpler brown seaweeds, so far as known, multiply only by means of zoöspores, which may grow directly into new plants, or, as has been observed in some species, two zoöspores will first unite. A few, like Ectocarpus (Fig. 28, A), are simple, branched filaments, but most are large plants with complex tissues. Of the latter, a familiar example is the common kelp, “devil’s apron” (Laminaria), often three to four metres in length, with a stout stalk, provided with root-like organs, by which it is firmly fastened. Above, it expands into a broad, leaf-like frond, which in some species is divided into strips. Related to the kelps is the giant kelp of the Pacific (Macrocystis), which is said sometimes to reach a length of three hundred metres.
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,