Fig. 70.—Fucus vesiculosus. A Portion of thallus with swimming bladders (a) and conceptacles (b). B Section of a female conceptacle; h the mouth; p the inner cavity; s oogonia. C Antheridiophore; a antheridium; p sterile cells. D Antheridia out of which the spermatozoids are escaping. E Fertilisation. F Germinating oospore.
Fig. 71.—Fucus serratus. a Portion of a male plant which has been exposed to the action of the open air for some time; small orange-yellow masses, formed by the antheridia, are seen outside the mouths of the male conceptacles (nat. size). b Cross section through the end of a branch of a female plant, showing the female conceptacles (× 4).
Fig. 72.—Sargassum bacciferum. A portion of the thallus, natural size.
Order 1. Fucaceæ. The following species are common on our coasts: Fucus vesiculosus (Fig. [70]) has a thallus with an entire margin, and with bladders arranged in pairs; F. serratus (Fig. [71]) without bladders, but with serrated margin; Ascophyllum nodosum has strap-like shoots, which here and there are swollen to form bladders; Halidrys siliquosa has its swimming bladders divided by transverse walls; Himanthalia lorea, which is found on the west coast of Norway, and the south coast of England, has a small perennial, button-shaped part, from the centre of which proceeds the long and sparsely branched, strap-like, annual shoot, which bears the conceptacles. The Gulf-weed (Sargassum bacciferum, Fig. [72]) is well known historically from the voyage of Columbus; it is met with in large, floating, detached masses in all oceans, and is found most abundantly in the Atlantic, off the Canary Islands and the Azores, and towards the Bermudas. The stalked, spherical air-bladders are the characteristic feature of this genus. The thallus is more highly developed than in Fucus, and there is a contrast between the stem and leaf-like parts. The portions which are found floating are always barren, only those attached are fertile.
Uses. The Fucaceæ, like the Laminariaceæ, are used as manure (the best kinds being Fucus vesiculosus and Ascophyllum nodosum), for burning to produce kelp, and as food for domestic animals (Ascophyllum nodosum is especially used for this purpose).
Class 9. Dictyotales.
The plants in this class are multicellular, and brown, with apical growth, new cells being derived either from a flat apical cell, or from a border of apical cells. The thallus is flat, leaf- or strap-shaped, attached by haptera, which are either found only at the base, or on the whole of the lower expansion of the thallus. The cells are differentiated into the following systems of tissues: an external, small-celled layer of assimilating cells, generally one cell in thickness, and an internal, large-celled layer of one or only a few cells in thickness, forming the mechanical and conducting tissues. All the reproductive cells are motionless. Asexual reproduction by naked, motionless spores (tetraspores) which are formed 1–4 in each tetrasporangium, the latter being outgrowths from the surface cells of special, sexless individuals. Zoospores are wanting. The sexual organs are of two kinds, oogonia and antheridia, which are formed from the surface cells, either on the same or different individuals. The oogonia are spherical or oval, and are generally placed close together; each contains one oosphere, which on maturity is ejected into the surrounding water, and is then naked and motionless. The antheridia are formed of longitudinal cells, united in groups, whose contents by repeated divisions—transverse and longitudinal—are divided into a large number of small, colourless, motionless spermatia—round or elongated—which are set free by the dissolution of the wall of the antheridium. The process of fertilisation has not yet been observed.