For some time now I have been fascinated by slime molds. I wrote about them in an earlier column, as an example of the new kingdom of living things known as protists or protoctists, but I did not go into detail. Today it's my job to try to make you share my fascination.

At first sight, that won't be easy. A slime mold has a form and lifestyle as disgusting as its name. It is a thin, sluggish film of living material with no fixed shape. It slowly creeps about, usually on the floor of a forest. It prefers dark, damp places, and for preference it eats decaying branches and old tree stumps and rotting leaves, although occasionally it will dine on a live fungus.

The slime mold becomes more interesting when we ask, what exactly is it? Some animals, such as the amoeba that most people meet in a first course in biology, are single-celled animals. Other creatures, like you and me, are multi-celled animals, collections of large numbers of cells in which different types of cells perform very different functions. The cells of your brain and the cells of your liver, for example, do not much resemble each other in what they do.

The cells of a slime mold start out in life like amoebas, individual but identical single-celled creatures, each with its own ability to eat, move, and reproduce. In fact, in some slime molds, such as one with the catchy name of Dictyostelium discoideum, individual members actually are amoebas. Each one wanders around, finding and eating its own food (usually bacteria) until the supply becomes scarce. At that point, one of the amoebas - and it can be any one - sends out a chemical signal. We even know what this chemical is; it is a substance known as cyclic AMP. Its effect is to attract other amoebas in the neighborhood. They flow together, lose their integrity as separate creatures, and many thousands of them unite to become a slime mold. Curious behavior indeed. H.G. Wells and Julian Huxley, writing about this seventy years ago, described it in a way I can't resist quoting: "Imagine that whenever two people meet each other in the street they run together into one blob ... so that ultimately the whole population of a town is rolled up into a gigantic mass of living substance that creeps about like a single creature; that is the sort of thing that happens as a matter of course in the life-history of a slime mold."

That's not the end of the story, however. The strange part is still to come. The creeping film wanders about, seeking and absorbing food. However, after a while it takes on a definite shape. Part of it hardens and forms itself into a stalk. At the top of that stalk a round mass forms. This is full of spores, which are ejected by filaments like coiled springs, blown away on the breeze, and land somewhere else. If that somewhere else is a place with suitable sources of food, the spores develop into new individual amoebas, which start the whole process over. Here are Wells and Huxley again: "It is almost as if a lump of silly putty were suddenly to pull itself together and model itself into a working clock." (Writing for an English readership, they actually said "plasticine" rather than "silly putty.")

Now we come to the question that most interests me. Clearly, the individual amoebas do not have any form of intelligence, as we understand the word. How do they know what to do? How do they communicate with each other? It has to be completely through chemical messengers.

In more complex life forms, such as insects, those chemical messengers have a name. They are called pheromones. Pheromones were originally thought to be related mainly to the chemical signals associated with sexual attraction between males and females of a species, and hence to be connected only with reproduction. Now, chemical messengers are discovered to be important in many areas of cooperative effort. For instance, in the first stages of constructing a termites' nest, termite larvae carry and drop small pieces of dirt. Each piece is impregnated by a pheromone generated by the termite larva. This pheromone has the property that it attracts other termite larvae, each carrying its own bit of dirt. The mound-building process has begun - but again, it has done so without any requirement that an individual larva possess some form of intelligence.

It is tempting to generalize, and to assert that when it comes to communication, chemical messengers are far more important than any other kind (such as the sounds that we make, or the color patterns on flowers that attract bees and other insects). This may well be equally true of humans. "Love at first sight" should perhaps be more accurately "love at first smell." But of course we would not know this, any more than the termite or the slime mold "knows" that it is sending out chemical signals that dictate the behavior of other members of the species.

That line of thought leads naturally to another, but it is not one I am sure I wish to pursue too far.

Look at the slime mold. Its individual members carry on with their separate lives, until they have completely exhausted the resources of the place where they live. They then indulge in a massive cooperative effort, building a giant structure (giant, at least, by their standards). This will be used to launch a few chosen members off in specially constructed vessels (the spores), seeking a new home with new and hopefully abundant resources. The individual members of the slime mold, of course, do not possess brains. They have no idea at all what is going on. Somehow, the knowledge of what to do is a property not of the individual part, but only of the whole.

Does this make any of you think of the space program, maybe a few hundred years from now? How much of what we do is decided by that super-slime mold, the human race, but remains unknown to any of its individual members?

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