Fig. 69.—Laminaria digitata (much reduced in size).
Order 16. Laminariaceæ. The thallus is more or less leathery, and has generally a root-like lower part (Fig. [69]) which serves to attach it, and a stalk or stem-like part, terminated by a large leaf-like expansion. Meristematic cells are situated at the base of the leaf, and from these the new leaves are derived. The older leaf thus pushed away by the intercalary formation of the younger ones, soon withers (Fig. [69]). Gametes are wanting. Zoosporangia are developed from the lower part of a simple, few-celled sporangiophore, which is an outgrowth from a surface-cell and has a large club-formed apical cell. The sporangia are aggregated into closely packed sori, which cover the lower part of the terminal leaf, or occur on special, smaller, lateral, fertile fronds (Alaria). Most of the species belonging to this order live in seas of moderate or cold temperature and occur in the most northern regions that have yet been explored, forming their organs of reproduction during the cold and darkness of the arctic night. Laminaria is destitute of a midrib and has only one terminal leaf. L. digitata has a broad leaf, which, by the violence of the waves, is torn into a number of palmate strips (Fig. [69]). L. saccharina has a small, undivided leaf. Alaria has a midrib and special fertile fronds. A. esculenta occurs plentifully on the west coast of Norway and on the shores of Great Britain. Chorda filum, a common seaweed, is thick, unbranched, and attains a length of several metres, without any strong demarcation between stalk and leaf. Some attain quite a gigantic size, e.g. Macrocystis pyrifera, whose thallus is said sometimes to be more than 300 metres in length. The Lessonia-species, like the above, form submarine forests of seaweed on the south and south-west coasts of South America, the Cape, and other localities in the Southern Hemisphere.
Uses. The large Laminarias, where they occur in great numbers, are, like the Fuci, used for various purposes, for example, in the production of iodine and soda, and as an article of food (Laminaria saccharina, Alaria esculenta, etc.). Laminaria saccharina contains a large quantity of sugar (mannit) and is in some districts used in the preparation of a kind of syrup; in surgical operations it is employed for the distension of apertures and passages, as for instance the ear-passage. It is by reason of the anatomical peculiarities and structure of the cell-walls, that they are employed for this purpose. The cell-walls are divided into two layers, an inner one which has very little power of swelling, and an outer one, well developed and almost gelatinous—the so-called “intercellular substance”—which shrivels up when dried, but can absorb water and swell to about five times its size. The stalks of Laminaria clustoni are officinal.
Order 17. Cutleriaceæ. The thallus is formed by the union of the originally free, band-shaped shoots. The growth is intercalary. Sexual reproduction by the conjugation of male and female gametes. An asexual generation of different appearance, which produces zoospores, arises from the germination of the zygote. Cutleria, Zanardinia.
Sub-Family 2. Acinetæ.
Branched, simple cell-rows with intercalary growth. The organs of reproduction are partly uni-and partly multicellular; in the unicellular ones a cell without cilia is formed, which may be destitute of a cell-wall, but has one nucleus (oosphere?), or which has a cell-wall and contains several (generally four) nuclei (aplanospores?); in the multicellular, monosymmetric swarm-cells with two cilia (spermatozoids?) are formed. The fertilisation has not been observed.
Order 1. Tilopteridaceæ. Haplospora, Tilopteris.
Family 2. Cyclosporeæ.
The individuals are multicellular, with growth by an apical cell. The thallus—often bilateral—is differentiated into a root-like structure (attachment-disc), and stem, sometimes also into leaves (Sargassum). Sometimes a differentiation occurs into various tissue-systems, viz. an external assimilating tissue, a storing tissue, a mechanical tissue of thickened, longitudinal, parenchymatous, strengthening cells, and a conducting tissue of sieve-cells, or of short sieve-tubes with perforated walls. Colouring material, as in Phæosporeæ. Vegetative reproduction can only take place by means of detached portions of the thallus (Sargassum), which are kept floating by means of bladders (Fig. [70] A, a, Fig. [72]). Zoospores are wanting.
The sexual reproduction takes place by oogamous fertilisation. The oogonia and antheridia are formed inside special organs (conceptacles), and are surrounded by paraphyses. The conceptacles (Fig. [70] B, Fig. [71] b) are small, pear-shaped or spherical depressions, produced by a special ingrowth of the surface cells of the thallus, and their mouths (ostioles) project like small warts; they are either situated near the end of the ordinary branches of the thallus (Fucus serratus, Fig. [71] a) which may be swollen on this account (Fucus vesiculosus, Fig. [70] A, b), or on special short branches (Ascophyllum, Sargassum). The vertical section of a conceptacle is seen in Fig. [70] B (see also Fig. [71] b) where, in addition to the paraphyses, oogonia only are seen (F. vesiculosus is diœcious—male plant, yellow-brown; female plant, olive-brown); but in some species antheridia, together with oogonia, are produced in the same conceptacle. The oogonia are large, almost spherical cells, situated on a short stalk, in each of which are formed from 1–8 (in Fucus, 8; in Ascophyllum, 4; in Halidrys, 1; in Pelvetia, 2) rounded, immotile oospheres. The wall of the oogonium ruptures, and the oospheres, still enclosed in the inner membrane, are ejected through the mouth of the conceptacle, and float about in the water, being finally set free by the bursting of the inner membrane. The antheridia are oblong cells (Fig. [70] C, a), many of which are produced on the same branched antheridiophore (Fig. [70] C); the numerous spermatozoids are provided with 2 cilia and are very small (Fig. [70] D, two antheridia surrounded by spermatozoids, one being open). The spermatozoids, still enclosed by the inner membrane of the antheridium, are similarly set free, and fertilisation takes place in the water, numerous spermatozoids collecting round the oosphere (Fig. [70] E), which is many times larger, and by their own motion causing it to rotate. After fertilisation, the oospore surrounds itself with a cell-wall and germinates immediately, attaching itself (Fig. [70] F) to some object, and by cell-division grows into a new plant.