QUESTION: I read about plants that are nematode-resistant and about beneficial nematodes, but nobody says what they are. I know they are microscopic, but microscopic what?

ANSWER: Worms, specifically roundworms: smooth, unsegmented, narrowly cylindrical invertebrates, which are pointed at both ends. There are a great many species - somewhere around 15,000 - and there could be over a million individuals in a square yard of healthy soil.

Some nematodes are parasites, which is beneficial when they live on pests, harmful when they live on plants. And those that live on animals, including pinworms, hookworms and trichinae, have caused untold human misery. But most soil-dwelling nematodes are benign, free-living creatures that aid in the recycling of plant and animal debris.

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