Part 22

GENERA: Empusa has a good many species which are parasitic on flies, moths, grasshoppers, plant-lice. The conidia emit a germ-tube which pierces the skin of the insect; a number of secondary conidia are then produced inside its body, by division or by gemmation similar to that taking place in yeast, each of which grows and becomes a long unbranched hypha, and these eventually fill up the body of the animal, causing distension and death. Each of these hyphæ projects through the skin, and abstricts a conidium, which is ejected by a squirting contrivance. The best known species is E. muscæ (Fig. 85), which makes its appearance epidemically towards autumn on the common house-fly, and shows itself by the dead flies which are found on the windows and walls attached by their probosces, distended wings, and legs. They have swollen abdomen, broad white belts of hyphæ between the abdominal rings, and are surrounded by a circle of whitish dust formed by the ejected conidia.--Entomophthora sends out, at definite places, from the mycelium hidden in the insect’s body, bundles of hyphæ, which serve the purpose of holding fast the dead insects, the ramifications attaching themselves to the substratum: the conidiophores are branched, the conidia are ejected by the divisional walls between the hyphæ and the conidia dividing into two layers, those which terminate the hyphæ suddenly expanding and throwing the conidia into the air. E. radicans makes its appearance epidemically on caterpillars.

=B.= Asexual reproduction by zoospores or conidia.

Family 2. =Chytridiales.=

In this family the mycelium is very sparsely developed or is wanting. The entire plant consists principally or entirely of a single zoosporangium whose zoospores have generally one cilium. The resting-spores arise either directly from the zoosporangium, which, instead of forming zoospores, surrounds itself by a thick cell-wall; or they are formed by the conjugation of two cells (in which case they are spoken of as oospores). Microscopic Fungi, parasitic on water plants (especially Algæ) or small aquatic animals, seldom on land plants.

Order 1. =Olpidiaceæ.= Without mycelium. Swarmspores and resting-spores.

In the Olpidieæ, the swarmspores, probably, most frequently form themselves into a plasmodium (naked mass of protoplasm) which may become a single zoosporangium or a resting sporangium. Olpidium trifolii occurs in Trifolium repens.--In the Synchytrieæ the plasmodium emerging from the swarmspores breaks up either at once, or after a period of rest, into smaller plasmodia, each of which will become a zoosporangium. Synchytrium anemones is found on Anemone nemorosa; S. mercurialis on Mercurialis perennis; S. aureum on many plants, particularly Lysimachia nummularia.

Order 2. =Rhizidiaceæ.= Mycelium present. Zoospores and resting-spores.

Chytridium (Fig. 86). Obelidium (Fig. 87) is bicellular; the one cell is the mycelium, the other the zoosporangium; found on insects. The species of Cladochytrium are intercellular parasites on marsh plants. Physoderma.

Order 3. =Zygochytriaceæ.= Mycelium present. Zoospores and oospores. The latter are the product of the conjugation of two cells (Fig. 88).

Polyphagus euglenæ on Euglena viridis. Urophlyctis pulposa on species of Chenopodium.

Family 3. =Mycosiphonales.=

The mycelium is bladder-like or branched. Zoospores. Sexual reproduction by oospores, which are produced in oogonia. The latter are fertilised, in some forms, by the antheridium.

Order 1. =Ancylistaceæ.= The entire bladder-like mycelium is used for the construction of zoosporangia, oogonia, or antheridia. Lagenedium is parasitic on Spirogyra, etc.

Order 2. =Peronosporaceæ.= Almost entirely parasites. The unicellular, often very long and abundantly branched mycelium lives in the intercellular spaces of living plants, especially in the green portions, and these are more or less destroyed and deformed in consequence. Special small branches (suction-organs, “haustoria”) are pushed into the cells in order to abstract nourishment from them. Both oospores and conidia germinate either immediately, or they develope into sporangia with swarmspores, having always two cilia. Only one oospore is formed in each oogonium; its contents (Fig. 89) divide into a centrally placed egg-cell and the “periplasm” surrounding it; this is of a paler colour and on the maturity of the oospore forms its thick, brown, external covering.

The Potato-fungus (Phytophthora infestans) is of great interest. Its thallus winters in the Potato-tuber; other organs for passing the winter, such as oospores, are not known. When the tuber germinates, the Fungus-hyphæ penetrate the young shoot and keep pace with the aerial growth and development of the plant. The conidiophores emerge through the stomata, especially on the under side of the leaves; they branch like a tree (Fig. 90), and appear to the naked eye as a fine mould on the surface of the plant. The disease soon makes itself known by the brown colouring of those parts of the plant which are attacked, and by their withering. An ovoid conidium arises at first by the formation of a dividing wall at the apex of each branch of the conidiophore (Fig. 90 c c), and immediately underneath it another is formed, which pushes the first to one side, and so on. These conidia sometimes germinate directly, and form a mycelium, but most frequently their protoplasm divides into many small masses, each of which becomes a pear-shaped zoospore provided with two cilia (Fig. 91). Water is required for their germination, and when the ripe conidia are placed in a drop of water the swarm-cells are formed in the course of about five hours. They swarm about in rain and dewdrops in the Potato-fields, and are carried with the water to the Potato-plants and to the tubers in the soil. The wind also very easily conveys the conidia to healthy Potato-fields and infects them. The enormous quantity of conidia and swarm-cells that may be formed in the course of a summer explains the rapid spreading of the disease; and the preceding makes it clear why wet summers are favourable to its existence. When the swarm-cells germinate, they round off, and then surround themselves with a cell-wall which grows out into the germ-tube, and pierces through the epidermis of the host-plant (Fig. 92). Having entered the host, a new mycelium is formed. The potato disease, since 1845, has been rampant in Europe; it has, no doubt, been introduced from America, which, it must be remembered, is the home of the Potato-plant.

The conidia exhibit various characters which are employed for the separation of the genera. Pythium is the most simple form. The contents of the terminally-formed conidia emerge as a spherical mass and divide into swarmspores. P. de Baryanum lives in the seedlings of many different Flowering-plants, which it completely destroys.--Phytophthora is distinguished by the circumstance that the sparsely-branched conidiophores bear, sympodially, chains of conidia. Besides the Potato-fungus (see above), Ph. fagi belongs to this group; it developes oospores very abundantly, and does great harm to seedlings of the Beech, Sycamore, and Pine trees.--Peronospora generally has conidiophores which are repeatedly forked, and bear a conidium on each of the most extreme ramifications. Many do great harm to their host-plants. P. viticola, on Vines, and P. nivea, on umbelliferous plants, have swarmspores, which are absent in the following species of this genus: P. sparsa, on Roses; P. gangliformis, on composites; P. alsinearum, on Stitchwort; P. parasitica, on cruciferous plants; P. viciæ, on Vetches and Peas; P. schachtii, on Beets; P. violacea, on the flowers of Scabiosa; P. radii, on the ray-florets of Matricaria.--Cystopus (Albugo) has the conidia developed in chains, which form a cohesive white layer underneath the epidermis of the host-plant. Cystopus candidus, on cruciferous plants, especially Shepherd’s Purse and Brassica; the germination commences on the cotyledons, and from this point the mycelium developes together with the host-plant; C. cubicus, on the leaves of Compositæ.

Order 3. =Saprolegniaceæ=, Water-Fungi which live as saprophytes on organic remains lying in water, for instance, on dead flies (Fig. 93), worms, remains of plants; but they may also make their appearance on living animals, being frequently found, for example, on the young trout in rearing establishments.

The thallus is a single, long and branched cell. It has one portion which serves as root, and lives in the substratum, where it ramifies abundantly for the purpose of absorbing nourishment; and another portion projecting freely in the water, and sending out hyphæ on all sides (Fig. 93). The asexual reproduction takes place by swarmspores (Fig. 94), which are developed in large sporangia; these swarmspores generally possess two cilia, and on germination grow into new plants. The entire protoplasm in the oogonium is formed into one or more oospheres, without any surrounding “periplasm.” The oospheres may not be fertilised (p. 100), and then develope parthenogenetically.

Genera: Saprolegnia, whose swarmspores disperse immediately after having left the sporangium. S. ferax is the cause of a disease in fish (“Salmon disease”) and in the crayfish.--Achlya, whose swarmspores accumulate in a hollow ball before the mouth of the sporangium.--Leptomitus has strongly indented hyphæ, causing a “linked” appearance. L. lacteus is frequent in the waste matter from sugar factories.--Monoblepharis deviates from the others by the greater development of its fertilising process; the oosphere, situated in an open oogonium, becoming fertilised by self-motile spermatozoids, which are provided with a cilium at the posterior end.

Class 2. =Mesomycetes.=

The Mesomycetes are intermediate forms between the Phycomycetes and the Higher Fungi. In the vegetative organs, and in the multicellular hyphæ, they resemble the Higher Fungi; the methods of reproduction, however, show the characters of the Phycomycetes, namely sporangia and conidiophores of varying size and with varying number of spores; definite and typically formed asci and basidia are not present. Sexual reproduction is wanting. The HEMIASCI are transitional between the Phycomycetes and the Ascomycetes, the HEMIBASIDII (Brand-Fungi) form the transition to the Basidiomycetes.

Sub-Class 1. =Hemiasci.=

The Hemiasci are Fungi with sporangia which, although resembling asci, yet have not, however, a definite form and a definite number of spores. Besides endospores, conidia, chlamydospores and oidia are found.

Order 1. =Ascoideaceæ.= Ascoidea rubescens forms irregular, reddish-brown masses in the sap issuing from felled Beeches. It has free sporangia, which resemble asci in their structure, in the development and ejection, and in the definite shape and size of the spores. The formation of the sporangia takes place when the nutriment is nearly exhausted, and resembles that of the conidia, since they are developed from the end of a hypha which enlarges, and the swelling becomes separated by a transverse wall. Within the sporangia numerous spores of a cap-like form are developed, which are set free through an opening at the apex. Sporangia are formed successively at the apex of the same hypha, the second commencing to develope as the first is dehiscing. Conidia and sporangia are not formed simultaneously; the former may be considered as closed sporangia.

Order 2. =Protomycetaceæ.= Protomyces pachydermus causes hard swellings on the stems and leaf-stalks of the Cichorieæ (Taraxacum, etc.). These swellings consist of chlamydospores (resting-spores), which germinate and become free, ascus-like sporangia, with numerous small spores. In nutritive solutions the chlamydospores form conidia with yeast-like buddings. P. macrosporus on Ægopodium, and other Umbelliferæ.

Order 3. =Thelebolaceæ.= Thelebolus stercoreus, is found on the dung of deer, hares, and rabbits, and has closed sporangia, which resemble asci in their shape and regular construction, and in the ejection of spores. The covering encloses only one sporangium, even where the sporangia arise close together.

This order, by reason of the covering of the sporangia, forms the transition from the Hemiasci to the Carpoasci, while the two first supply an intermediate step to the Exoasci.

Sub-Class 2. =Hemibasidii, Brand-Fungi.=