Weird Plant-Animals

Near the bottom of life’s pyramid there is a weird race of plant-animals. They are among the closest of all many-celled living things to the primaeval protoplasm from which all life arose.

They are the slime molds found on decaying logs and tree stumps in damp woods or on piles of rain-soaked dead leaves in shady gardens. The nightmarish mycetozoa—botanists call them myxomycetes—are timeless survivals out of living creation’s dank, warm cradle. Some of the weirdest imaginings of malevolent life on other planets picture it in the form of gigantic slime mold aggregations—undifferentiated masses of naked protoplasm endowed with a malign intelligence which has evolved without the intermediaries of nervous systems or brains.

These organisms can be considered one of nature’s probing experiments towards higher forms of life. The experiment was a failure, but unlike most of nature’s discards these organisms have survived. Even now they may be engaged in a process of evolution all their own.

Biologists are not entirely agreed in which kingdom to place the organisms, although they usually are classified with the plants. They start life as spores, like the dust of molds or toadstools whose single-celled particles serve the same reproductive function as seeds in higher plants. From each spore arises from one to four animal-like organisms, hardly distinguishable from the one-celled protozoan animal, the amoeba. Each swims about freely for a time by means of tentacle-like arms, the flagellae.

These free-moving living particles are known as “swarm cells”. Each is an individual with a film-like skin separating it from all other individuals. That is, the protoplasm of each cell is enclosed within a boundary and in the center of each is a nucleus. These one-celled “animals” wander about freely for a few days. During this time they may mate, as individuals. More commonly each loses its flagellae and splits into several fragments. Each of these fragments becomes a complete organism. These mate, with complete fusion of their bodies. The result is a double plant or animal—depending on whether it is observed by a botanist or zoologist—known as a zygote. The fragments are extremely voracious little creatures devouring greedily the one-celled plants, or bacteria, which they encounter.

When the fusion is complete the zygote, in turn, starts to split up into single-celled organisms but after a few divisions hundreds of these single-celled animals coalesce into a tiny ball, like the seed pod of a plant. In a few days thousands of these spheroids collect into a so-called “plasmodium”. The hitherto individual pseudo-protozoans meanwhile have lost their cell walls. The primaeval substance of millions is mixed together into a slimy mass full of cell nuclei. This is an aggregation of “naked protoplasm”. It is hardly to be compared with the body of any higher plant or animal where each cell retains something of its individuality, however closely its activities may be coordinated with those of its fellows in the same community. The mass proceeds to behave like a voracious animal. It moves and feeds as a unit and apparently with a purpose. Within the naked protoplasm there is apparently some incomprehensible sense of fellowship which eventually evolves into consciousness and intelligence, developing nerve and brain on the way upwards. It would be hazardous to say that this evolution could have taken no other path.

From the central body great numbers of thread-like filaments are sent out to penetrate the substance of rotting wood or the surface of a dead leaf. These threads seem to be like an army’s scouting parties, pushed ahead to locate supplies when advancing troops are living off the country. When a supply is found they are drawn in and the whole slimy organism acts once more as a coordinated whole.

The plasmodium moves forward steadily for about 50 to 60 seconds, pauses for a few moments, and then reverses itself and creeps backward, but never quite so far as it previously had gone ahead. Then, after another pause, it crawls forward again. Thus there is an overall slow advance and at the bottom of life the slime molds lay down the pattern of progress recapitulated in human societies and civilizations as well as in the lives of individual men and women. They merit consideration in the philosophy of history.

The advancing mass of raw protoplasm acts like an animal and grows like an animal as it ingests food, with constant splitting of the cell nuclei which it contains. There are vacuoles within the protoplasm in which the food particles are ingested. They then are digested by means of enzymes (body chemicals), as in higher animals.

Such a plasmodium can be taken from its damp habitat and dried. Then it will roll up into a ball and pass into a resting stage from which it will revive completely in a few hours when supplied with moisture again. The ball may keep its vitality for several years.

Some species pass as much as a year in the active plasmodium stage, and some a few days. At the end of this phase of its existence the mass of raw protoplasm breaks up into fragments—sometimes as many as a hundred. Then, as the process is described for one common species “in an hour or two each of these fragments has risen into a pear-shaped body with a narrow base, a dark stalk being just apparent through the translucent white substance.” In about six hours the black, hair-like stalk has grown to its full length and bears at its top a young “sporangium” consisting of a globule of viscous plasma with a diameter about a fifth the length of the stalk. This globe is about the size of a mustard seed and ranges in color from pure white through golden-yellow, light crimson, violet, purple and black.

A pink flush now begins to pervade the sporangium caused by the formation of branching threads. The nuclei in the plasma still present the same appearance as those observed in the streaming plasmodium. In about another hour these nuclei show the beginning of division. As this process develops the plasma becomes separated in masses of two spores capacity. An hour later the nuclei have divided and the young spores are forming. Their color rapidly changes. In about the first twenty hours after the first concentration of the fragments of the plasmodium they have matured and present the appearance of minute black pins standing in regular order on wood. The ripe fruit, or sporangium, then dries and breaks.

On placing the spore in water its membranous wall slips off and the naked contents lie for several hours without apparent change in an ellipsoid form. Constriction then takes place and the ellipsoid splits into one to four globular bodies adhering together and exhibiting slow amoeboid movements. Each globular body now develops a flagellum—a long, whip-like extension, and the cluster swims away by means of these flagellae.

Now the whole life process is ready to be repeated. There are more than 400 species of these slime molds and they are distributed over all the temperate and tropic zones. If only the spores and the stalked little ball containing them are considered, the slime mold would be placed squarely in the kingdom of plants. But when the protoplasm escapes from the spore and starts moving about ingesting bacteria, the behavior is that of a one-celled animal. When the cells unite to form a plasmodium there is a close likeness to a many-celled animal.