Order 20. Corallinaceæ. Partly crustaceous, partly erect, branched Algæ, thickly incrusted with lime, so that a few species (Lithothamnia, also called Nullipora) occur in fossilized condition from Jurassic to Tertiary periods. Melobesia, Lithophyllum, Lithothamnion, Corallina.
Uses. “Carragen” is the thallus of Chondrus crispus (Irish Moss) and Gigartina mamillosa. It is a common article of food on the coasts of Ireland, and swells to a jelly when cooked. It is officinal. Rhodymenia palmata is generally eaten as food in Ireland and in some places on the west coast of Norway; it is also used as food for sheep and hence is termed “Sheep-seaweed.” Agar-Agar is the jelly obtained from species of Gelidium and Gigartina growing in China and Japan.
Sub-Division III. FUNGI.
Mode of Life. The Fungi have no chlorophyll, and are thus unable in any stage of their existence to assimilate carbon; they must therefore live as saprophytes or parasites. There is, however, no strong line of demarcation between these; many Fungi commence as true parasites, but only attain their full development upon or in dead plants or animals (Rhytisma, Empusa). Many saprophytes may occasionally appear as parasites, and are then designated “facultative parasites” (Nectria cinnabarina, Lophodermium pinastri), in contradistinction to those which only appear as parasites, “obligate parasites” (Mildew, Brand-and Rust-Fungi, Cordyceps).
The parasites which live on the surface of the host-plant are termed epiphytic (Mildew, Fusicladium); and those living in its tissues are termed endophytic (Ustilago, Peronospora). Epizoic (Oidium tonsurans, Laboulbenia) and endozoic Fungi (Cordyceps, Entomophthora), are distinguished, in the same manner, as those which live on the surface or in the interior of animals. The Fungi designated pathogenic are especially those which produce disease in human beings and in animals.
Most of the diseases of plants are attributed to the parasitic Fungi. These force their way into the host-plant by piercing the outer wall of the epidermis, as in the Potato-disease; or by growing in through the stomata, e.g. the summer generations of the Rust of Wheat; or they can only penetrate through a wound, e.g. Nectria. Some effect an entrance into the host-plant by the secretion of a poisonous matter or ferment, which softens and destroys the cell-walls (Sclerotinia). Some Yeast and Mould Fungi secrete ferments (enzymes), which, for example, convert cane-sugar into a sugar capable of fermentation.
The relation of the parasitic Fungus to the host-plant is mainly of two kinds. In the one case, the cell-contents are destroyed, the protoplasm is killed, and the cellular tissue becomes discoloured and dies (Peronospora, Armillaria mellea, Polyporus); in the other case, the parasite has an irritating effect on the cellular tissue, whereby the affected organ grows more rapidly and becomes larger than normal, producing hypertrophy. Such malformations are termed Fungi-galls (Mycocecidia); in this manner “witches’ brooms” are produced by Æcidium, “pocket-plum” by Taphrina, and other deformities by Exobasidium and Cystopus candidus. This hypertrophy may either be produced by a vigorous cell-multiplication, which is most frequently the case, or by the enlargement of the individual cells (Synchytrium, Calyptospora). The relation between host and Fungus among the Lichens is of a very peculiar nature, termed “symbiosis.”
Vegetative Organs. The vegetative parts of a Fungus are termed its mycelium.[10] This is formed of a mass of long, cylindrical, branched cells resembling threads (and hence termed hyphæ), which have a continued apical growth. The mycelium, in its early development, shows a well-marked difference between the two main groups of true Fungi: in the Phycomycetes, or Algal Fungi, the mycelium has no transverse walls, and is therefore unicellular, while in the Mesomycetes and Mycomycetes it is provided with dividing walls, which gradually arise during growth, in the youngest hyphæ; intercalary transverse walls may also be formed at a later period. In the hyphæ of some of the Higher Fungi (Hymenomycetes), connections may be formed between two contiguous cells of the same hypha, by a protuberance growing out from an upper cell just above the transverse wall, and forming a junction with the cell below. These are known as clamp-connections; they appear to be of use in affording communication between the two cells.
The hyphæ of Fungi, where they come in contact with one another, often grow together, so that H-formed combinations (fusions) are produced, which give rise to very compact felted tissue. When the hyphæ are not only closely interwoven, but also united and provided with many transverse walls, the mycelium assumes the appearance of a tissue with isodiametric cells, and is then termed pseudo-parenchyma. The hyphæ-walls are sometimes very much thickened, and composed of several layers, and the external layers, by the absorption of water, may often swell very much and become mucilaginous. In some instances the walls are colourless, in others coloured, the most frequent colour being brown. The cell-contents may also be coloured, and in that case are generally yellow; this colour is chiefly connected with the fat (oil) which may be found in abundance in the Fungi, whilst starch is invariably absent in all the true Fungi.
The mycelium assumes many different forms; sometimes it appears as a thread-like, cobwebby, loose tissue, less frequently as firm strands, thin or thick membranes, horn-like plates or tuber-like bodies. The thread-like mycelium may, in the parasitic Fungi, be intercellular or intracellular, according as it only extends into the interstices between the cells or enters into the cells proper. In the first case there are generally found haustoria, or organs of suction (e.g. among the Peronosporaceæ; Taphrina, on the contrary, has no haustoria); but haustoria are also found among the epiphytic Fungi (e.g. Erysiphaceæ). Intracellular mycelia are found in the Rust-Fungi, in Claviceps purpurea, Entomophthora, etc. In spite of its delicate structure, this mycelium may live a long time, owing to the circumstance that it continues to grow peripherally, while the older parts gradually die off (“fairy rings”).