Sandra Smith, age 12, of South Portland, Maine, for her question:

Are slime molds plants or animals?

As a rule, we can tell the plants from the animals with no trouble at all. We do not confuse the lion with the dandelion, the crow with the crocus or the pecan with the toucan. But certain small living things puzzle even the experts. The slime molds are classified , as both plants and animals.
The small slithery slime molds live very secret lives and many types refuse to perform their life cycles in captivity. Countless varieties may exist, but so far only about 406 species have been identified. No more than 30 have been studied in complete detail under laboratory conditions. The fascinating study of slime molds is claimed by both plant and animal researchers. And they are classified in both these branches of biology. As animals, they are classified in the order Mycetozoa of the single celled phylum Protozoa. As plants, they are classified in their own phylum, Myxophyta.
Each of these strange living things dwells in a borderline world between the plant and animal kingdoms. It leads a double life. Through one phase, it slithers and slides, moving around in search of food like an animal. And like an animal, it devours and digests solid food. But when time comes to multiply, the mobile hunter behaves like a plant. Its jellylike blob of slithery protoplasm solidifies into a fixed sheet of glassy material and then sprouts masses of spore cases. The powdery spores break loose and scatter on the winds, somewhat like the spores of the fungus plants.
The slime molds have been called the fungus animals. They are harder to find during their animal phase because they shun light and seek moisture. They hide in the soil, in rotting logs and often under the bark of a shady tree. Each is a blob of naked cytoplasm. It may be less than an inch to several inches wide and, depending on the type, it may be grayish~or tinted with a pale, pastel color.
The jellified blob may be a zygote, or double cell. Most often it is a blend of many zygotes working in harmony. Biologists call it a plasmodium and it moves like a spreading ink blot, engulfing bacteria, spores and scraps of decaying vegetation as it goes. As a rule, it streams along, slows down and reverses direction every minute or so. At top speed the slithery hunter can travel five inches in about a minute. Biologists are not sure when or why the plasmodium decides to end its phase of animal existence.
When time comes, the slime mold comes to the surface. Within a few hours, it becomes a miniature garden crowded with colorful spore cases. The crowded pods may be as short as threads of velvet or almost an inch tall, depending upon the species. They come in shapes like cups or saucers, fronds or feathers, pears or olives, bats or balls. The crowded garden may be rosy pink or primrose yellow, orange or chalky white, brown or glistening purple. The glassy pods often sparkle like rainbows. But you cannot admire their shapes and colors without the help of a strong magnifying glass.
When the pods burst their hard shells, the microscopic spores rest awhile. The powdery specks may rest in this dormant stage through 50 years. With shade and dewy moisture they come to life and each grows a double tail or tiny threads. It becomes a swarm cell and the miniature acrobat begins to twist and twirl, curl and coil through the filmy moisture. When it meets another sprightly swarm cell, the two unite. They give up their tails and the double cell becomes a zygote. Now the plasmodium phase begins again. More zygotes merge with the jellified blob and the mobile mass of slime mold grows until it is ready for its vegetable phase of reproduction.