Part 16

The class is divided into two families:--

1. PHÆOSPOREÆ: 1 Sub-Family, Zoogonicæ; 2 Sub-Family, Acinetæ.

2. CYCLOSPOREÆ: Fucaceæ.

Family 1. =Phæosporeæ.=

The family consists of multicellular plants, whose cells are firmly united together to form a thallus; this, in the simplest cases, may be a branched filament of cells (Ectocarpus), or, in the highest, may resemble a stem with leaves (Laminariaceæ), while all transitional forms may be found between these two. The thallus grows by intercalary divisions (e.g. Ectocarpus), or by an apical cell (e.g. Sphacelaria); pseudo-parenchymatous tissue may sometimes be formed by cells, which were originally distinct, becoming united together. The size of the thallus varies; in some species it is quite small--almost microscopical,--while in the largest it is many metres in length.

The vegetative cells in the lower forms are nearly uniform, but in those which are more highly developed (Laminariaceæ and Fucaceæ), they are sometimes so highly differentiated that mechanical, assimilating, storing and conducting systems may be found; the last named systems are formed of long cells with perforated, transverse walls, which bear a strong resemblance to the sieve-tubes in the higher plants.

The colouring matter in the living cells (“phæophyl”) contains chlorophyll; but this is concealed by a brown (“phycophæin”), and a yellow (“phycoxanthin”) colouring material, and hence all these Algæ are a lighter or darker yellow-brown. Starch is not formed. Asexual reproduction takes place, (1) by zoospores which arise in unilocular zoosporangia, and are monosymmetric, with two cilia attached laterally at the base of the colourless anterior end (Fig. 65), the longer one being directed forwards and the shorter backwards; or (2) by aplanospores (?).

Sexual reproduction has only been discovered in a few cases, and takes place by means of gametes (oogamous fertilisation perhaps occurs in the Tilopteridæ). The gametes have the same structure as the zoospores, and arise in multilocular gametangia; these, like the zoosporangia, are outgrowths from the external surface, or arise as modifications from it. The conjugating gametes may be similar (e.g. Ectocarpus pusillus), or there may be a more or less pronounced difference of sex, an indication of which is found in Ectocarpus siliculosus (Fig. 66). When the gametes in this species have swarmed for a time, some, which are generally larger, are seen to attach themselves by one of the cilia, which by degrees is shortened to form a kind of stalk (compare the upper gamete in Fig. 66 II); these are the female gametes, which now become surrounded by a number of males endeavouring to conjugate with them, but only one succeeds in effecting fertilisation. The protoplasm of the two gametes coalesces (Fig. 66 III), and a zygote (e) is formed. The male gametes which do not conjugate may germinate, but the plants derived from them are much weaker than those produced by the zygotes. Strongly pronounced sexual differences are found in the Cutleriaceæ, in which order the male and female gametes arise in separate gametangia (Fig. 67 A). The male gametes (Fig. 67 D) are much smaller than the female gamete (Fig. 67 C); the latter, after swarming for a short time, withdraws the cilia, and is then ready to become fertilised (Fig. 67 B, E), thus we have here a distinct transition to the oogamous fertilisation which is found in the Fucaceæ. Alternation of generations is rarely found.

1. Sub-Family. =Zoogonicæ.=

Reproduction by means of gametes and zoospores.

Order 1. =Ectocarpaceæ.= The thallus consists of single or branched filaments with intercalary growth, extending vertically from a horizontal, branched filament or a disc, but sometimes it is reduced to this basal portion only. Zoosporangia and gametangia (for fertilisation see Fig. 66) are either outgrowths or arise by the transformation of one or several of the ordinary cells. The most common genera are: Ectocarpus and Pylaiella.

Order 2. =Choristocarpaceæ.= Choristocarpus, Discosporangium.

Order 3. =Sphacelariaceæ.= The thallus consists of small, parenchymatous, more or less ramified shoots, presenting a feather-like appearance. In the shoots, which grow by means of an apical cell (Fig. 68 S), a cortical layer, surrounding a row of central cells, is present. Sporangia and gametangia are outgrowths from the main stem or its branches. Sphacelaria, Chætopteris are common forms.

Order 4. =Encoeliaceæ.= Punctaria, Asperococcus, Phyllitis fascia.

Order 5. =Striariaceæ.= Striaria, Phlœospora.

Order 6. =Dictyosiphonaceæ.= Dictyosiphon.

Order 7. =Desmarestiaceæ.= Desmarestia aculeata is common.

Order 8. =Myriotrichiaceæ.= Myriotrichia.

Order 9. =Elachistaceæ.= Elachista fucicola is a common epiphyte on species of Fucus.

Order 10. =Chordariaceæ.= The shoot-systems are often surrounded by mucilage. Chordaria; Leathesia difformis occurs as rounded, brown-green masses of the size of a nut, generally attached to other Seaweeds.

Order 11. =Stilophoraceæ.= Stilophora rhizodes is common.

Order 12. =Spermatochnaceæ.= Spermatochnus paradoxus is common.

Order 13. =Sporochnaceæ.= Sporochnus.

Order 14. =Ralfsiaceæ.= Ralfsia verrucosa is common as a red-brown incrustation on stones and rocks at the water’s edge.

Order 15. =Lithodermataceæ.= Some species of the genus Lithoderma occur in fresh water.

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.