Professor Bruce Ing talked to us on "The Natural History of Slime Moulds", a comparatively less well-known group of fungi which are in fact not strictly fungi at all but share some of the characteristics of protozoa, a group of animals of the simplest type, each consisting of a single cell. They might better be termed "honorary fungi" perhaps. Their somewhat forbidding name belies their fascination and beauty as we came to see later in the talk. I think that up to this point most of us, knowing little about them, had equated Slime Moulds with Myxomycetes. It was now pointed out to us that the Myxomycetes are only one type of Slime Mould but by far the most attractive so far as Bruce is concerned.
There has always been much disagreement about the classification and relationships of these organisms exhibiting as they do some of the characteristics of both the plant and the animal kingdoms. Because early collectors found them in the same situations as fungi, being particularly abundant in forested areas where they appear in great profusion on dead and decaying wood, or wood litter, and on dead leaves, they not unnaturally assumed that they were fungi. However they do not share the roles and functions of fungi and physiologically they are both different from and similar to fungi at various stages of their development. As regards role and function, Myxomycetes do only very minor damage to living plants so they cannot be truly classed as parasites nor can they be accurately described as saprophytes for they feed on living bacteria and on fungal sporophores, spores, and mycelium as well as bits of non-living organic matter. Physiologically, the acellular, creeping, phase of the slime moulds is definitely animal-like. However the reproductive structures are plantlike, producing spores covered by definite walls.
Biologists have always tended to dismiss them and Bruce voiced his gratitude that this was so and that, despite all the above discrepancies, they were ultimately consigned to the kingdom of mycology, albeit in a sub-division of their own, since he enjoyed working with mycologists so much.
His interest in the group was aroused forty three years ago when as an undergraduate he was examining mosses and found what looked like small chestnut coloured grapes among them. His lecturer at the time did not know what these were and could not help. However Bruce subsequently came across the Little Guide to Models of Mycetozoa published by the Natural History Museum and also Lister’s book on the group, all of which went a considerable way to explain his finds and he became completely captivated by them. Unfortunately at the time no-one else in Britain shared his enthusiasm although the Americans and Dutch were interested. But today the study of these organisms is no longer an obscure pursuit. There are three International Conferences on Myxomycetes attended by 60 to 80 delegates from all over the world and the Japanese have a Myxomycete Society.
Bruce went on to describe the reproductive cycle of the Myxomycetes.
The Myxomycetes bear their spores inside a fruitbody covered by a peridium. The spores are generally spherical with a definite rather thick cell wall which can be smooth, spiny, warty or reticulate. The thickness of the wall makes them exceptionally resistant to unfavourable conditions, especially to long periods of dessication and they have been known to germinate after 61 years of storage in a herbarium. Most species will germinate in water. In nature spores of the Myxomycetes probably germinate in rain water which has formed a dilute solution with the substrate on which the spores happen to lie. A spore germinates by one of two methods. It either cracks open, or germinates by means of a tiny pore.. The method seems to be specific for individual species. Either a myxamoeba or a flagellated swarm cell emerges. To a certain extent this depends on the environment. If the spores are suspended in water the emerging protoplasts are often flagellated from the very beginning. Sometimes, however, an amoeba will issue from the spore, remain quiescent for a few minutes, and then develop flagella. However although flagellated cells are characteristic of the Myxomycetes they are not necessary for the completion of the life cycle of all species.
After a swarm cell escapes from the spore case, it swims about with a rapid rotary movement which is combined with amoeboid contractions until after a period of mobility it withdraws its flagella, thus changing into a myxamoeba. When food is abundant and environmental conditions are favourable, myxamoebae divide repeatedly, giving rise to a large population of haploid (containing one set of chromosomes) cells. These haploid cells finally fuse to form a diploid zygote ( a union of two reproductive cells finally containing two sets of chromosomes). As the zygote grows its nucleus gradually becomes transformed into a multinucleate amoeboid structure, the plasmodium.
Being a mass of protoplasm the plasmodium does not have a definite size or shape. At one time it is globose, at another it is flat and sheet-like spreading over a large area in the form of a very thin network which is often brilliantly coloured. Ever changing, ever flowing, the plasmodium creeps over the surface of the substrate, engulfing particles of food in its way. Slime mould plasmodia are of various colours, ranging from colourless to white, grey, black ,violet, blue, green, orange, and red, the colour depending in part upon the species. The yellow and the white plasmodia are probably the most commonly encountered.
When conditions are favourable the plasmodium finally develops into one or more fruiting bodies. This transformation is accompanied by cleavage of the protoplasm into uninucleate portions which become enveloped by walls and eventually mature into spores. The type of fruiting body varies with the species and to some extent with environmental conditions prevailing during the development of the fructification. Three types of fruitbody are produced.
In the first of these the plasmodium forms numerous individual stalked sporangia generally crowded together on the portion of the substrate formerly occupied by the plasmodium. Each sporangia has a peridium of its own. Examples of slime moulds which produce sporangia are Hemitrichia clavata, Physarum globuliferum, Physarum viride and various species of Stemonitis, Comatricha, and Arcyria.
The second type of fructification is called aethalium. In contrast, this is, a black sooty, fairly large, sometimes massive, generally cushion-shaped structure with little, if any, evidence of individual sporangia. The entire body is enclosed in a peridium.
Some common examples of aethaloid fructifications are Lycogala epidendrum, Tubifera ferruginosa and various species of Fuligo.
The third type of fructification, known as a plasmodiocarp, is similar to a stalkless sporangium, but differs in that it retains to a certain extent the branching habit of the plasmodium. An example of this is Hemitrichia serpula.
The sporangial type of fructification may be stalked or sessile. If the stalks extend into the sporangium, the portion inside the sporangium is known as the columella. However, many stalkless sporangia also possess a columella.
The presence and type of capillitium are important characteristics in the classification of the Myxomycetes. The capillitium is a group of non-living, hair-like structures which may unite to form an intricate network attached to the columella or to the peridium, or which may consist of simple or branched filaments, unattached and independent of each other.
Soon after the formation of capillitium, spore delimitation begins. In a number of species the capillitium is so constructed as to be a definite aid in the dissemination of the spores. The entire protoplasm within the fruitbody is consumed in the development of the resting spores. In a mature fruitbody they occur closely packed in between the capillitial threads and are liberated when the peridium disintegrates to be dispersed mainly by the wind.
The talk concluded with a selection of slides as follows, demonstrating the enormous variety of environments in which Myxomycetes flourish.
Rotten logs may be a good source of finds, though less so when invaded by Honey Fungus. One log pile was a particularly good illustration of a good habitat. Further species were found growing on both Stereum hirsutum and its host, dead beech. Another had invaded Phlebia but in this case was confined to the area of the fungus. A section of oak bark was very dry so not so good as it should have been. Sycamore bark has been known to support as many as ten species of cortical myxomycetes.
Some slime moulds are associated with green algae. Beach litter which is nice and damp is a rich source of species. Straw heaps, though not bales, hold moisture and also provide a wealth of interesting finds. However this habitat is fast disappearing. Boulder surfaces which support liverworts are a prime habitat for Myxomycetes. The damp Atlantic woodlands of Western Britain and Snowdonia where these abound are both excellent sites for the group.
In contrast and at first, unexpectedly, Myxomycetes are also to be found growing in desert conditions. This is because desert plants, e.g. Cacti, are able to retain moisture internally. Rotten Opuntias are a wonderful host for these organisms which grow inside the rotting pads.
In such high mountainous areas as the Jura many new species have appeared at the snow-line. These are most prolific where snow has lain for at least three months. The Cairngorms in Scotland where the snow is very acid have also proved fruitful. Some species are to be found growing on the remains of dead herbaceous stems of bilberry and on the grass on Ben Lawers. Lepidoderma species have been found on pine needles. In Spain, others have developed on living Juniper which has just emerged from the snow.
Finally some common species which most of us are likely to come across include Lycogala terrestre, a spring species with a pink plasmodium not to be confused with Lycogala epidendrum which has a red plasmodium; the Arcyria genus with loofah like structures; the easily identifiable bright orange Trichia decipiens, and Trichia varia the commonest species with a brilliant white plasmodium, oozing white drops of milk. Stemonitis fusca and Fuligo septica are also comparatively easily recognisable and very common in Britain.
