The Myxomycetes, or Slime Moulds

Despite their vital role in soil ecology, the Myxomycetes go largely unnoticed by field naturalists due partly to their small size and unpredictable occurrence, but perhaps also to their absence from, or token appearance in, field guides.

Fig 2. Life cycle of a slime mould.

Fig 1. Plasmodium of Badhamia sp .

Slime moulds only become visible to the patient observer at the fruiting stage. After fusion of compatible haploid myxamoebae the newly formed diploid cell undergoes multiple nuclear divisions without cell division to form a plasmodium (the 'slime' in slime mould – Fig. 1), which eventually migrates to the surface of the soil or log, or up the stems of plants to produce visible and often brightly coloured sporocarps, and to release their spores into the air. On germination, each spore releases a single myxoflagellate, this is a single cell with two flagella, one long and motile, the other usually short and rigid. These feed by ingesting bacteria, and divide repeatedly until under suitable conditions they absorb their flagella to become myxamoebae, and so the process continues (Fig.2).

Although most commonly sought in woodlands, Myxomycetes can be found almost anywhere in damp conditions after rain, and even around melting snowfields in the mountains. Most are no more than a few millimetres tall, some much smaller, but many species produce large clusters of sporocarps together, and many are brightly coloured, making them easier to find.

Fig 3. Structure of a typical sporangium.

The Myxomycete fruit body or Sporocarp typically consists of a single sporangium which may be supported by a stalk, or may be sessile (Fig. 3). The stalk may extend into the sporangium to form a columella, and the sporangium may also contain a capillitium, a network of more or less flexible threads or elaters, which support the spores and may assist in their dispersal. These threads may be in the form of separate elaters, as in  Trichia (Fig. 4), or may form a reticulate network as in  Stemonitis (Fig. 5), for example.

The sporangium is covered by a surface skin of one to 3 layers, the peridium, which disrupts to release the spores. The form and nature of the peridium and capillitium, together with the size and ornamentation of the spores, are important features for the identification of slime moulds. Individual sporangia may fuse to produce sausage shaped bodies called plasmodiocarps, or in some species many sporangia cluster together to form aethalia, in which individual sporangial walls are lost.

Fig 4 Capillitium and Spores of  Trichia

Fig. 5 Capillitium of  Stemonitis axifera

One of the largest fruit bodies belongs to  Enteridium (or  Reticularia ) lycoperdon (Fig 6), once thought to be a variety of puffball. Its aethalia form silvery cushions up to 10 cm across, usually on dead standing trees in Spring, especially alder, but also on large fallen branches and trunks, and even on worked timber such as window frames. As it matures, the silvery covering becomes whiter, and eventually ruptures to release a mass of cocoa brown spores.

Arcyria obvelata  (Fig 7) produces stalked, cylindrical yellow sporangia up to 15 mm tall on fallen wood, often crowded together, which when mature look like tiny loofahs. The "spongy" part of the loofah is the capillitium - a network of elastic threads or elaters marked with prominent part-rings which support the spores and aid their dispersal. The capillitium is supported by a small yellow cup (the calyculus) at the top of the short stalk, which is the remains of the peridium, the membranous skin which covers the developing sporangium.

Fig. 8. Aethalium of  Fuligo septica var. flava.

Fig. 9. Aethalia of Lycogala terrestre

Fig. 11  Mucilago crustacea

Fig. 6. Aethalium of  Enteridium lycoperdon

Fig. 7. Sporangia of  Arcyria obvelata

Fig 10. Sporangia of  Leocarpus fragilis

Mucilago crustacea (Fig 11) can be found encrusting plant stems and leaves with a white spongy mass of fused sporangia called an aethalium, which may be up to several cm long. The white colour is caused by lime in the sporangium walls, which dehisce to release a mass of black spores, but it is the immature creamy white or pale yellow plasmodial slime that gives this species its common name of "Dog's Vomit"!

The best time to look for slime moulds is a few days after a period of rain, when plasmodia can migrate into the open air and form fruit bodies without risk of drying out too quickly. As the fruit bodies, or sporocarps, mature, they dry out to release their powdery spores. In many cases, the capillitium, or elaters, expand on dehiscence to facilitate spore dispersal.

Fig 12. Developing pseudoaethalium of Tubifera ferruginosa, emerging from a white plasmodium.