Hanneke Frank, age 8, of Rockford, 111.0 ors question:

How do plants breathe under water?

The world has had plants for many hundred million years. But let’s use our imaginations to speed up this long story into six days. From Monday until. Friday, all plants lived in the sea and used oxygen from the water. On Saturday morning a few came on land and learned to use the oxygen in the air.

All the lush and lovely land plants are descended from ancestors that lived in the sea. The plant world used oxygen and other gases in the water long before it used the gases of the air. The living cells in a plant must exchange gases with the outside world, but a plant does not breathe by puffing in and out. Nature has a special postal service to deliver the gases a plant needs and collect up its waste gases.

This busy postal service is called diffusion and it works in both air and h*ater. You can see how diffusion works when you watch a puff of pearly smoke spread through the air of a room: Gas molecules are too small for our eyes to see, but they spread by diffusion, seeping through walls and tiny holes. Diffusion carries air to and from the inside cells of a land plant. The same postal service carries water to and from the cells of a water plant.

The cells take oxygen and other gases they need from the seeping air or water. The waste gases they do not need seep outside again. In plants we call this process respiration because it is somewhat different from our breathing. The respiration of land and water plants is done by diffusion. But a land plant has special cells to let the postman in and out.

A leaf is covered with a skin called the epidermis . It is a layer of cells fitted together like the rocks in a stone wall.  An oak leaf has pores called stomates in the epidermis and air seeps back and forth through these tiny doorways.    The top side of a water lily leaf is in the air and it too has stomates. But there are no stomates on the soggy underside of the leaf.    Seaweeds and other plants that live right in the water have no stomates.

But the diffusion postman carries the gases these plants need. Water soups right through the epidermis cells and the water carries dissolved oxygen and other gases. A tossing wave grabs a few molecules of gas when it swoops through the air. Waterfalls and rushing rapids add more air molecules to the water. As water diffuses through the cells, they take the gases they need and the waste gases they do not need seep outside again.

The mermaid weed is a plant which can live under water oz' rooted in soggy ground with its head in the air. St can arrange its skin cells to exchange gases with the air or the water. But most land plants can no longer take the gases they need from the water as their ancestors did. Respiration stops and they choke if they are swamped in deep water.