SLIME MOULDS

Sometimes, after soaking summer rains, you can find a yellow, white or pink frothy slime on a log or stump or even smothering a plant on the forest floor. This is a Slime Mould moving to a site where it can produce its fruitbodies. Most of their lives, slime moulds are hidden inside well-rotted logs or stumps, or buried in leaf mould. When it’s time to fruit, however, slime moulds migrate to a better site for spore dispersal. The slimy mass of protoplasm, the plasmodium (Fig. 2) , can travel several feet and climbs any object living or dead that gives it a site advantage for spore dispersal. So, it's not unusual to find slime moulds fruiting on green plants, dead twigs, old polypores, stumps, logs or head high up a living tree trunk!

The slime stage doesn't help much in identifying species as they often look much alike at this phase. When they fruit, however, we can separate species by the shape, size, and colour of the fruitbody and spores. Many of the common species can be identified using a simple 10 X hand lens (loupe). For serious students of the group, however, microscopic characteristics are important. For those who have a microscope, there is a useful book called "How to Know the True Slime Moulds " by Marie Farr (Published by Brown, Dubuque, Iowa) that gives keys, descriptions and illustrations for most of the slime moulds in North America.

There are four types of fruitbodies in slime moulds. The commonest is the sporangium. Here, all the protoplasm separates into pieces and each piece forms a tiny fruitbody. Sporangia are usually only a few mm tall but in some exceptional species, such as Stemonitis, they may reach several cm in height. Hundreds or even thousands of sporangia, more or less the same size and shape, can be produced simultaneously from a single plasmodium.

The aethalium is a relatively large fruitbody and is very variable in size. Some, such as Fuligo septica, form crusty masses more than 15 cm long. Because of their large size, only one or a few aethalia are produced at a time. In Lycogala the aethalia are smaller, usually a cm or less, and are sometimes produced in large numbers. In such cases the aethalia may be mistaken for sporangia but there is a large variability in size of the fruitbodies. In Tubifera ferruginosa, the "aethalium" is composed of a large number of individual sporangia that appear to be fused together. This type of fruitbody is called a pseudoaethalium. The prefix pseudo- is often used in mycology and means false.

The plasmodiocarp consists of a network of fat veins over the surface of a log or stump. Hemitrichia serpula is the most common species of slime mould producing this type of fruitbody.

Slime Moulds are the stepchild of Mycology

Slime moulds don't fit easily into our classification system. They move and feed like animals. They engulf all kinds of organic particles in their path like giant amoebae. They digest what they can and, animal-like, they violently eject unwanted particles.  Slime moulds are fungus-like, however, in producing tiny fruitbodies containing spores which are dispersed by wind. There are about 500 species known, and many of these have a global distribution indicating that slime moulds have been around for a long time.

Slime Moulds are not fungi but they have been adopted by mycologists and most studies on the taxonomy and biology of slime moulds, and the books and articles written on these topics, have been carried out by mycologists.

In many fundamental aspects of their structure and biology Slime Moulds are not remotely related to the true Fungi.  In Whitakers 'Five Kingdom' system Slime Moulds are not in the Kingdom Fungi but are placed in the Protoctistans.   A few of these differences are as follows.

The nuclei in the slime mould assimilative stage (plasmodium) are diploid.   In the true fungi the nuclei in the hyphae are haploid or dikaryotic.

The fungus protoplasm is protected by a hard wall of chitin, chitosan, glucan, mannan.  The slime moulds thallus has no wall and the plasmodium is protected by mucus. 

The spore in slime moulds is protected by a wall of cellulose!  Cellulose is not found in fungi. It is the stuff of plants and probably some protozoans.

The slime mould spore can germinate to produce an amoeba (= myxamoeba, haploid) that can grow divide and pultiply like protozoan amobae.  Fungi have no amoeboid stage.

Slime moulds feed amoeba-like by phagocytosis.  The plasmodium, which can be very large, flows over and engulfs organic particles that, animal-like, it either digests or physically ejects. Fungi feed by extracellular sectretion of enzymes.  These enzymes break down the substrate into simpler water-soluble compounds that are then absorbed through the hyphal wall.

The slime mould plasmodium  can move en masse from one point to another to feed or to fruit.  Slime mould plasmodia can migrate several feet up a plant to fruit.  Fungal hyphae grow but don't move.