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Mycetozoa
1911 Encyclopedia Britannica
or (Myxomycetes, Schleimpilze), in zoology, a group of organisms reproducing themselves by spores. These are produced in or on sporangia which are formed in the air and the spores are distributed by the currents of air. They thus differ from other spore-bearing members of the animal kingdom (which produce their spores while immersed in water or, in the case of parasites, within the fluids of their hosts), and resemble the Fungi and many of the lower green plants. In relation with this condition of their fructification the structures formed at the spore-bearing stage to contain or support the spores present a remarkable resemblance to the sporangia of certain groups of Fungi, from which, however, the Mycetozoa are essentially different.
Although the sporangial and some other phases have long been known, and Fries had enumerated 192 species in 1829, the main features of their life-history were first worked out in 18 591860 by de Bary (r and 2). He showed that in the Mycetozoa the spore hatches out as a mass of naked protoplasm which almost immediately assumes a free-swimming flagellate form (zoospore), that after multiplying by division this passes into an amoeboid phase, and that from such amoebae the plasmodia arise, though the mode of their origin was not ascertained by him.
The plasmodium of the Mycetozoa is a mass of simple protoplasm, without a differentiated envelope and endowed with the power of active locomotion. It penetrates the interstices of decaying vegetable matter, or, in the case of the species Badhamia utricularis, spreads as a film on the surface of living fungi; it may grow almost indefinitely in size, attaining under favourable conditions several feet in extent. It constitutes the dominant phase of the life-history. From the plasmodium the sporangia take their origin. It was Cienkowski who (in 1863) contributed the important fact that the plasmodia arise by the fusion with one another of numbers of individuals in the amoeboid phase - a mode of origin which is now generally recognized as an essential feature in the conception of a plasmodium, whether as occurring among the Mycetozoa or in other groups (7). De Bary clearly expressed the view that the life-history of the Mycetozoa shows them to belong not to the vegetable but to the animal kingdom.
The individual sporangia of the Mycetozoa are, for the most part, minute structures, rarely attaining the size of a mustardseed, though, in the composite form of aethalia, they may form cake-like masses an inch or more across (fig. 2 I). They are found, stalked or sessile, in small clusters or distributed by the thousand over a wide area many feet in diameter, on the bark of decaying trees, on dead leaves or sticks, in woods and shrubberies, among the stems of plants on wet moors, and, generally, at the surface in localities where there is a substratum of decaying vegetable matter sufficiently moist to allow the plasmodium to live. Tan-heaps have long been known as a favourite habitat of Fuligo septica, the plasmodia of which, emerging in bright yellow masses at the surface prior to the sporangial (in this case aethalial) phase, are known as " flowers of tan." The film-like, expanded condition of the plasmodium, varying in colour in different species and traversed by a network of veinlike channels (fig. 5), has long been known. The plasmodial stage was at one time regarded as representing a distinct group of fungi, to which the generic name Mesenterica was applied. The species of Mycetozoa are widely distributed over the world in temperate and tropical latitudes where there is sufficient moisture for them to grow, and they must be regarded as not inconsiderable agents in the disintegrating processes of nature, by which complex organic substances are decomposed into simpler and more stable chemical groups.
Classification
The Mycetozoa, as here understood, fall into three main divisions. The Endosporeae, in which the spores are contained within sporangia, form together with the Exosporeae,. which bear their spores on the surface of sporophores, a natural. group characterized by forming true plasmodia. They constitute the Euplasmodida. Standing apart from them is the small group of the mould-like Sorophora, in which the amoeboid individuals only come together immediately prior to sporeformation and do not completely fuse with one another.
A number of other organisms living on vegetable and animal bodies, alive or dead, and leading an entirely aquatic life, are included by Zopf (31) under the Mycetozoa, as the " Monadina," in distinction from the " Eumycetozoa," consisting of the three groups above mentioned. The alliance of some of these (e.g. Protomonas) with the Mycetozoa is probable, and was accepted by de Bary, but the relations of other Monadina are obscure, and appear to be at least as close with the Heliozoa (with which many have in fact been classed). The limits here adopted, following de Bary, include a group of organisms which, as shown by their life-history, belong to the animal stock, and yet alone among animals' they have acquired the habit, widely found in the vegetable kingdom, of developing and distributing their spores in air.
Class Mycetozoa.
Sub-class I. - EuPLASMODIDA.2 Division 1. - Endosporeae. Cohort I. - Amaurosporales.
Sub-cohort i. - Calcarineae.
Order Physaraceae. Genera: Badhamia, Physarum, Physarella, Trichamphora, Erionema, Cienkowskia, Fuligo, Craterium,. Leocarpus, Chondrioderma, Diachaea. Order 2. Didymiaceae. Genera: Didymium, Spumaria, Lepidoderma. Sub-cohort 2. - Amaurochaetineae.
Order 1. Stemonitaceae. Genera: Stemonitis, Comatricha, Ener !
- , thenema, Echinostelium, Lamproderma, Clastoderma. Order 2. Amaurochaetaceae. Genera: Amaurochaete, Brefeldia. Cohort 2. - Lamprosporales.
Sub-cohort I. - Anemineae.
Order Heterodermaceae. Genera: Lindbladia, Cribraria, Dietydium. Order 2. Licaeceae. Genera: Licea, Orcadella. Order 3. Tubulinaceae. Genera: Tubulina, Siphoptychium, Alwisia.. Order 4. Reticulariaceae. Genera: Dictydiaethalium, Enteridium,. Reticularia. Order 5. Lycogalaceae. Genus: Lycogala. Sub-cohort 2. - Calonemineae.
Order 1. Trichiaceae. Genera :] Trichia, Oligonema, Hemitrichia,. Cornuvia. Order 2. Arcyriaceae. Genera: Arcyria, Lachnobolus, Perichaena. Order 3. Margaritaceae. Genera: Margarita, Dianema, Prototrichia, Listerella. Division 2. - Exosporeae. Order Ceratiomyxaceae. Genus: Ceratiomyxa. Sub-class 2.:-Sorophora.
Order I. Guttulinaceae. Genera: Copromyxa, Guttulina, Guttulinopsis. Order 2. Dictyosteliaceae. Genera: Dictyostelium, Acrasis, Polysphondylium. 1 Bursulla, a member of Zopf's Monadina, likewise forms its spores in air.
2 The classification of the Euplasmodida here given is that of A. and G. Lister (22), the outcome of a careful study of the group extending over more than twenty-five years. The writer of this article desires to express his indebtedness to the opportunities he has had of becoming familiar with the work of his father, Mr A. Lister, F.R.S., whose views on the affinities and life-history of the Mycetozoa he has endeavoured herein to summarize.
Life-History Of The Mycetozoa Euplasmodida Endosporeae. 'We may begin our survey of the life-history at the point where the spores, borne on currents of air, have settled among wet decaying vegetable matter. Shrunken when dry, they rapidly absorb water and resume the spherical shape which is found in nearly all species. Each is surrounded by a spore wall, sheltered by which the protoplasm, though losing moisture by drying, may remain alive for as many as four years. In several cases it has been found to give the chemical reaction of cellulose. It is smooth or variously sculptured according to the species. Within the protoplasm may be seen the nucleus, and one or more contractile vacuoles make their appearance. After the spore has lain in water for a period varying from a few hours to a day or two the wall bursts and the contained protoplasm slips out and lies free in the water as a minute colourless mass, presenting amoeboid movements (fig. 1, c). It soon assumes an elongated piriform shape, and a flagellum is developed at the narrow end, attaining a length equal to the rest of the body. The minute zoospore, thus equipped, swims away with a characteristic dancing motion. The protoplasm is granular within but hyaline externally (fig. I, d). The nucleus, lying at the end of the body where it tapers into the flagellum, is limited by a definite wall and contains a nuclear network and a nucleolus. It often presents the appearance of being drawn out into a point towards the flagellum, and a bell-like structure [first described by Plenge (27)], staining more darkly than the rest of the protoplasm, extends from the base of the flagellum and invests the nucleus (fig. 2, a and c). The other end of the zoospore may be evenly rounded (fig. 1, d) or it may be produced into short pseudopodia (fig. 1, e). By means of these U the zoospore captures bacteria which are drawn into the body and enclosed in digestive vacuoles. A contractile vacuole is also present near the hind end. Considerable movement may be observed among the granules of the interior, and in the large zoospores of Amaurochaete atra this may amount to an actual streaming, though without the rhythm characteristic of the plasmodial stage.
Other shapes may be a, After A. Lister.
FIG. 3. - Three stages in the division of the Zoospore of Reticularia Lycoperdon (X 1000).
Figures I, 4, and 11-22 are from the British Museum Guide to the British Mycetozoa. The other figures are from Lankester's Treatise on Zoology, part I. Introduction and Protozoa. Fascicle I. Article Mycetozoa.
mainly in the state of isolated amoebae. Pinoy finds that the amoebae of this group live on particular species of bacteria, and that the presence of the latter is a necessary condition for the development of the Sorophora, and even (as has been recognized by other workers) for the hatching of their spores. Pinoy's results indicate, though not so conclusively, that bacteria are likewise the essential food of the Euplasmodida in the early phases of their life-history. The zoospores do, however, ingest other solid bodies, e.g. carmine granules (Saville Kent, 15).
The zoospores multiply by binary fission, the flagellum being withdrawn and the nucleus undergoing mitotic division, with the formation of a well-marked achromatic spindle (fig. 3).
It is probable that fission occurs more than once in the zoospore stage; but there is not satisfactory evidence to show how often it may be repeated.2 At this, as at other phases of the life-history, a - resting stage may be assumed as the result of drying, but also from other and unknown causes. The flagel lum is withdrawn and the protoplasm, becoming spherical, secretes a cyst wall. The organism thus passes into the condition of a microcyst, from which when dry it may be awakened to renewed activity by wetting.
At the end of the zoospore stage the organism finally withdraws its flagellum and assumes the amoeboid shape. It is now known as an amoebula. The amoebulae become endowed, as was first recognized by Cienkowski, with mutual attraction, and on meeting fuse with one another. Fig. 4 represents a group of such amoebulae. Several have already united to form a common mass, to which others, still free, are converging. The protoplasmic mass thus arising is the plasmodium. The fusion between the protoplasmic bodies of the amoebulae which unite to form it is complete. Their nuclei may be traced for some time in the young plasmodium and no fusion between them has been observed at this stage (20). As the plasmodium increases in size by the addition of amoebulae the task of following the fate of the individual nuclei by direct observation becomes impossible.
The appearance of an active plasmodium of Badhamia utricularis, which, as we have seen, lives and feeds on certain fungi, is shown in fig. 5. It consists of a film of protoplasm, of a bright yellow colour, varying in size up to a foot or more in diameter. It is traversed by a network of branching and anastomosing channels, which divide up and are gradually lost as they approach the margin where the protoplasm forms a uniform and lobate border. Elsewhere the FIG. 5. - Part of the Plasmodium of Badhamia utricularis (X 8).
main trunks of the network may lie free with little or no connecting film between them and their neighbours. The plasmodia of other species, which live in the interstices of decaying vegetable matter, are less easily observed, but on emerging on the surface prior to 2 Pinoy states (26) that the spores of Spumaria alba, cultivated with bacteria on solid media, hatch out into amoebae, which under these conditions do not assume the flagellate stage. The amoeba from a spore was observed to give rise by three successive divisions to eight amoebulae.
After A. Lister.
FIG. I. 1 - Stages in the Hatching of the Spores of Didymium difforme. a, The unruptured spore.
b, The protoplasmic contents of the spore emerging. It contains a nucleus with the (light) nucleolus, and a contractile vacuole (shaded).
c, The same, free from the spore wall.
d, Zoospore, with nucleus at the base of the flagellum, and contractile vacuole.
e, A zoospore with pseudopodial processes at the posterior end, to one of which a bacillus adheres. Two digestive vacuoles in the interior contain ingested bacilli.
f, Amoeboid phase with retracted flagellum.
FIG. 2. - Zoospores of Badhamia panicea, stained (X 650).
In a and c the bell-like structure investing the nucleus is clearly seen.
temporarily assumed by the zoospore. Attaching itself to an object it may become amoeboid, either with (fig. 1, f) or without (fig. 2, c) the temporary retraction of the flagellum; or it may take an elongated slug-like shape and creep with the flagellum extended in front, with tactile and apparently exploratory movements.
