Part 19

Order 17. =Nemastomaceæ.= Furcellaria, which has dichotomously branched, round shoots, is common on the coasts of Great Britain.

Order 18. =Rhizophyllidaceæ.= Polyides, Rhizophyllis.

Order 19. =Squamariaceæ.= The Algæ belonging to this order form crust-like coverings on stones, mussel-shells, and on other Algæ, but are not themselves incrustated: Petrocelis, Cruoria, Peyssonellia.

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. 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”).

String-like mycelia may be found, for example, in Phallus, Coprinus, and are formed of hyphæ, which run more or less parallel to each other. Membrane-like mycelia are chiefly to be found in Fungi growing on tree-stems (Polyporaceæ and Agaricaceæ); they may have a thickness varying from that of the finest tissue-paper to that of thick leather, and may extend for several feet. The peculiar horny or leather-like strands and plates which, for instance, appear in Armillaria mellea, are known as Rhizomorpha; they may attain a length of more than fifty feet. The tuber-like mycelia or sclerotia play the part of resting mycelia, since a store of nourishment is accumulated in them, and after a period of rest they develope organs of reproduction. The sclerotia are hard, spherical, or irregular bodies, from the size of a cabbage seed to that of a hand, internally white or greyish, with a brown or black, pseudo-parenchymatous, external layer. Sclerotia only occur in the higher Fungi, and are found both in saprophytes, e.g. Coprinus, and in parasites, e.g. Claviceps (Ergot), Sclerotinia.

=Reproduction.= SEXUAL REPRODUCTION is found only among the lower Fungi which stand near to the Algæ, the Algal-Fungi, and takes place by the same two methods as in the Algæ, namely by conjugation and by the fertilisation of the egg-cell in the oogonium.

The majority of Fungi have only ASEXUAL reproduction. The most important methods of this kind of reproduction are the sporangio-fructification and the conidio-fructification.

In the SPORANGIO-FRUCTIFICATION the spores (endospores) arise inside a mother-cell, the sporangium (Fig. 80). Spores without a cell-wall, which move in water by means of cilia and hence are known as swarmspores or zoospores, are found among the Oomycetes, the sporangia in which these are produced being called swarm-sporangia or zoosporangia (Figs. 86, 87, 91, 94).

In the CONIDIO-FRUCTIFICATION the conidia (exospores) arise on special hyphæ (conidiophores), or directly from the mycelium. When conidiophores are present, the conidia are developed upon them terminally or laterally, either in a basipetal succession (in many Fungi, for example in Penicillium, Fig. 111, Erysiphe, Cystopus), or acropetally (in which method the chains of conidia are often branched; examples, Pleospora vulgaris, Hormodendron cladosporioides). All conidia are at first unicellular, sometimes at a later stage they become two-celled or multicellular through the formation of partition-walls (Piptocephalis). The conidia with thick, brown cell-walls, and contents rich in fats (resting conidia), can withstand unfavourable external conditions for a much longer period than conidia with thin walls and poor in contents.

The SPORANGIA arise either from the ordinary cells of the mycelium (Protomyces), or are borne on special hyphæ. They are generally spherical (Mucor, Fig. 80; Saprolegniaceæ), egg-, pear-, or club-shaped (Ascomycetes), more rarely they are cylindrical or spindle-shaped. While among the Phycomycetes the size, form, and number of spores are indefinite in each species, in the Ascomycetes the sporangia (asci) have a definite size, form, and number of spores. The spores of the Ascomycetes are known as ascospores.

The sporangio-fructification is found under three main forms.

1. FREE SPORANGIOPHORES which are either single (Mucor, Fig. 78), or branched (Thamnidium).

2. SPORANGIAL-LAYERS. These are produced by a number of sessile or shortly-stalked sporangia, being formed close together like a palisade (Taphrina, Fig. 105).

3. SPORANGIOCARPS. These consist usually of many sporangia enclosed in a covering, they are found only in the Carpoasci, and are also known as ascocarps. The parts of an ascocarp are the covering (peridium), and the hymenium, which is in contact with the inner wall of the peridium, and is generally made up of asci, and sterile, slender hyphæ. The latter either penetrate between the asci and are branched and multicellular (paraphyses, Figs. 103 d, 123, 125, 129), or clothe those parts of the inner wall which bear no asci (periphyses; among many peronocarpic Ascomycetes, e.g. Chætomium, Sordaria, Stictosphæra hoffmanni). The ascocarps are produced directly from the mycelium, or from a stroma, that is a vegetative body of various forms, in which they may be embedded (Figs. 116 B, C).

Among the conidio-fructifications there are, in the same way, three divisions.

1. FREE CONIDIOPHORES (Fig. 109). The form of the conidiophores, the shape, and number of its spores are various. In the most highly developed Fungi, the Basidiomycetes, there are, however, special more highly developed conidiophores, the basidia, which have a definite form and spores of a definite shape and number. The conidia borne on basidia are called basidiospores.

2. CONIDIAL-LAYERS. (a) The SIMPLEST case of this is found when the conidiophores arise directly from the mycelium, parallel to one another, and form a flat body (e.g. Exobasidium vaccinii, Hypochnus; among the Phycomycetes, Empusa muscæ and Cystopus). (b) In a HIGHER form the conidial-layers are thick, felted threads (stroma) inserted between the mycelium and the hymenium (i.e. the region of the conidiophores). Examples are found in a section of the Pyrenomycetes (Fig. 122). (c) The HIGHEST form has the basidial-layer, that is a conidial-layer with more highly developed conidiophores (basidia). The basidial-layer, with stroma, and the hymenium (region of the basidia), forms the basidio-fructification, which is branched in the Clavariaceæ, and hat-shaped in other Hymenomycetes (in these groups the hymenium is confined to the lower side of the pileus).

The hymenium of the conidial-layer and basidial-layer is composed entirely of conidiophores, or of conidiophores and sterile hyphæ (paraphyses) which are probably always unicellular. Paraphyses are found in Entomophthora radicans, and in certain Basidiomycetes (e.g. Corticium).

3. CONIDIOCARPS (pycnidia). A special covering surrounds the conidia-forming elements. The inner side of this covering (peridium) bears the hymenium, i.e. those elements from which the conidia are abstricted. The conidiocarps arise either immediately from the hyphæ or from a stroma in which they are generally embedded. Conidiocarps are entirely wanting in the Phycomycetes. On the other hand they are found among the Ascomycetes and Basidiomycetes, and in the latter group the conidiocarps contain more highly differentiated conidiophores (basidia) and are known as basidiocarps. Conidiocarps with simple conidiophores, are found only among the Basidiomycetes, in the Uredinaceæ, and in Craterocolla cerasi. In the Ascomycetes (Figs. 120 d, e; 117 a, b; 123 a; 124 b) the conidiocarps are visible, as points, to the naked eye, while the basidiocarps of the Basidiomycetes (Figs. 170, 171, 173–176, 178–180) vary from the size of a pea to that of a child’s head. The “spermogonia” of the Ascomycetes and Lichenes, are conidiocarps with small conidia (microconidia) which germinate sometimes more slowly than other conidia, and formerly were erroneously considered as male reproductive cells, and called spermatia.