Roger Lee Carmine, age 8, of Bellamy, Va., for his question:
What is pond scum?
A quiet pond is an invitation to all kinds of water loving plants. At a glance, we see a mass of floating greenery and maybe the water looks like a dark green mirror. When we look carefully, we see that the floating garden has a variety of different plants. In a sluggish stream, the bigger plants will be eel grass, water cress and maybe hyacinth. In the stagnant swamps, the bigger plants will be rushes and sedges cattails and mosses.
Among the tangle of wet green leaves there will be smaller specks of greenery, some of them too small to be identified. The water looks green because it teems with tiny alga plants, too small to be seen one by one. Along with diatoms, which are no bigger, these algae make the water rich with plankton. This is the favorite food of baby fish and inky black tadpoles.
On top of the water, sometimes all alone in the middle of a quiet pool, there may be a floating raft of scum. It is lovely and green in color, but slippery and slimy to touch. What's more, when you get hold of it, there seem to be n o leaves or stems. This pond scum turns out to be just a wad of wet, slimy greenery.
Actually, this wad of green carpeting may be composed of a variety of different plants. Most of them are green algae the simplest of all green plants. They have no roots, no stems and no flowers. The plant is a colony of green cells, all alike. Altogether we know of 5000 different green alga plants and a goodly number of them may be found floating on or in ponds.
One of the most common algae in pond scum is called Spirogyra. There are a great number of these strange little one celled plants, each with a slight variation. The soft green cells cling together to form long slippery threads. Under the microscope, we see that each cell contains a flat ribbon wound around in a graceful spiral shape – which is why this plant is called Spirogyra.
Another algae which forms pond scum is called Ulothrix. It is a vivid green color, and like the Spirogyra, its cells also cling together end to end to form long slippery threads. Under the microscope, we see that the cells are cylinders. Though no alga has a true root, the Ulothrix is often fixed to a solid object b y a holdfast at the end of a long thread. We often find it stuck to partly submerged stones and floating logs.
If you do not look closely, you may mistake a carpet of duckweed for pond scum. This is very insulting, for algae are the simplest plants and duckweed rates among the most advanced. For it is actually a flowering plant. True pond scum is a slimy mass with no leaves, no stem, no roots. Duckweed is a floating carpet of tiny leaves interlocked to look like a patch of clover. From the underside of the leaves, true roots grow down into the water. At the right time of year, tiny blossoms appear and the brilliant green carpet of duckweed appears to be dotted with snowflakes.