Dan Everist, age 13, of Sioux City, Iowa, for his question:

How do plants take in clean water from the ground?

Freshly dug carrots and potatoes tend to be rather grubby, but this dirt is on the outside. When we recall that plants absorb their water from the soil, it is natural to wonder why no dirty crumbs are found inside the leaves and stems, seeds and petals. But these internal plant cells are as clean as driven snow. The explanation for this is osmosis, the amazing process used to absorb moisture form the ground. The ground water diffuses, molecules by separate molecule, through the skin membranes that enfold the plant cells. Even a tiny speck of dirt is too big to pass through.

The filtering of ground water is done by membranes around the roots. No dirty dirt passes through into the plant. However, ground water contains an assortment of dissolved chemicals and some of these are necessary to the plant. These vital chemicals are in the form of tiny particles, invisibly dissolved and suspended in the ground water. And the root membranes can be somewhat selective. They admit the invisibly small chemical particles needed by the plant and keep out most of those that are useless and maybe dangerous.