Neither fully fungus nor animal, but both

Another Life: Season of mellow fruitfulness, indeed - so many apples the last are still ripening on the tree - but also of the necessary agents of decay and recycling: the damp-loving moulds, mildews, rots and fungi. Also, those very strange creatures, the slime moulds.

Munching a mildewed but crisp and juicy apple, I took a detour through our wind-blown huddle of trees, now laying themselves bare in a deep litter of ash and sycamore leaves. Plenty of fallen branches, too, from the black and brittle alders.

What I was hoping to spot (but didn't) has been variously described by readers, writing to "Eye on Nature" over the years, as "a soft, yellow, coral-like substance", "creamy yellow cylinders about three inches high, on blades of grass, and like thick cream to touch" or, simply, "scrambled egg".

Alternatively, there might have been "a grey-white gelatinous substance, amorphous in shape", "a blob of porridge" or even "like something an animal might have regurgitated". A slime mould can, indeed, look very like dog vomit, and in Ireland, seems more likely to catch the eye on somebody's lawn than in the woods. But since the prime foods of these organisms are bacteria and fungi on fallen leaves or rotting wood, I had hoped that our own moist little copse might have conjured some by now. It is probably still too young and too far upwind of the old Atlantic oakwoods along the coast: slime moulds and ancient forest go together. But I undoubtedly didn't look hard enough. In Co Offaly, the expert eye (and hand-lens, and probably microscope) of Dr Roland McHugh found no fewer than 80 species.

For a picture of "a slimy, pale grey mass that resembles cold porridge" sheathing a branch in a forest in Co Tyrone, try www.ulstermuseum.org.uk. This is Brefeldia maxima, which can grow very big indeed. A soil ecologist in Bristol University, Dr Alan Feest, once described his encounter with it in a way that conveys the essential otherness of slime moulds: "I came to what appeared to be a bucket of whitewash thrown over a large tree stump. [ It] was about two metres across, slimy and had a surface texture rather like that of a cauliflower. The whole object must have weighed several kilograms. A visit the next day showed that it had moved partly from the stump onto a nearby bramble, leaving a trail of slime where it had been. On the next day, the white slime had gone, only to be replaced by what appeared to be a thick dusting of cocoa powder." It had moved! What are we dealing with here? So peculiar is a slime mould, its nature almost depends on when you're looking at it. It is neither fungus nor animal but shares in the lifestyle of both.

Myxomycetes start out as microscopic single cells with one nucleus that hatch from spores blown in on the wind or eaten and excreted by insects (the "cocoa powder" above was the final fruiting, or spore-producing stage).

A SPORE CAN WAIT for decades to germinate until the moisture outside feels right and there's a whiff of bacteria for food. The cells can swim with whip-like flagellae, then grow into amoebae and these eventually meet up and fuse into the more visible stage called a plasmodium. This can be a blob a mere centimetre across, like those of the bright yellow "scrambled egg", or a mass a metre wide and containing millions of nucleii - but it is still one giant cell.

A one-centimetre plasmodium can crawl about 10cm in a day in search of food, the bigger ones even faster. How do they move? Their protoplasm is netted with veins that rhythmically contract and relax, pushing their fluid contents a little more forward than backward each time. One species of slime mould propels itself up tree trunks to feed on bracket fungi. It seems to be able to smell the fungus from some distance and engulfs it, leaving a slimy mass behind.

A plasmodium also has amazing shape-shifting powers. It can travel through anything its nucleii can pass through, including the "grain" of wood, so that it may enter one end of a log and emerge from the other.

Perhaps most uncanny of all, it can work things out. A scientist in Japan's Institute of Physical and Chemical Research chopped up a common yellow slime mould called Physarum polycephalum and separated the pieces into corridors of a miniature maze. At its entrance and exit, he put a block of jelly filled with nutritious ground oat-flakes.

The pieces of mould spread out and coalesced into a single organism. Then it withdrew from the dead-ends in the maze and formed a thick tube linking the entrance and exit, choosing the shortest of several possible routes between the two blocks of food.

Some porridge . . . some scrambled egg . . . some triffid.