PURSLANE'S A pesky garden weed until youstart to eat it, ten it's a treasure fo which you'll thank heaven whenever you find a patch growing in among the vegetables.

It spreads succulent stems and flattened oval leaves under upright crops like corn and peppers. Its growing season is short in the East -- it prefers the heat waves of mid-July and August. But give it a California summer, and it grows stems an inch thick.

Colonists probably brought purslane to this continent among their other pot herbs. "Miller's Garden Kalendar" of 1775 (conveniently reprinted by the National Capital Area Federation of Garden Clubs in 1971) counts the plant among other garden regulars sown in spring and harvested in summer.

We may have lost touch with that herbal tradition, but we haven't lost out on purslane. It still maintains itself, finding sunny bare lawn spots, cleared garden corners, even sprouting between the cracks of city sidewalks if given the chance.

Nibble the next batch you find, and you'll discover a crunchy, mild, slightly acidic green fit for the salad. Purslane can be cooked, too, but it tends to turn slimy. Pickled purslane maintains some crunch.

When people were a little more patient, they gathered purslane seed as well. Once plants mature, they bloom an inconspicuous flower, then produce a little round seed pod full of round black seeds comparable to those of the poppy. They are not all that hard to gather. Pick purslane as it blooms, then bring it inside and spread it on a dishcloth. In two or three days its seed pods will mature and pop open, and you can easily separate seeds from the larger green plant parts.

Use the seeds in baking -- as part of your grain or as a topping -- or use them in salad dressings, the perfect complement to a purslane slaw.

It's not just because purslane is free that we should learn to eat it. It's nutritious too. It has fewer calories, over twice the calcium, almost 10 times the iron and almost 20 times the vitamin A found in cabbage.

Not a bad record, for a weed. PURSLANE SLAW (5 servings) Dressing:

1/4 cup plain yogurt

1/4 cup mayonnaise

3 tablespoons lemon juice

1/8 teaspoon dry mustard

1/8 teaspoon salt

Sprinkling of seeds (purslane, poppy or caraway) Slaw:

1 cup purslane leaves and stems, coarsely chopped

1 cup cabbage and carrots, coarsely grated

1 large onion, minced

2 tablespoons fresh parsley, minced

Mix dressing ingredients together and pour over vegetables. PURSLANE PICCALILLI (Makes 1 quart)

3 cups purslane leaves and stems

1 large onion, thinly sliced

4 cloves garlic

Approximately 1/4 cup pickling spices (may include whole cloves, whole allspice, mustard seeds, cinnamon or nutmeg)

1 cup cider vinegar

1 cup sugar or 1/2 cup honey

Combine purslane, onion, garlic and spices in a stainless steel pan. Bring vinegar and sugar or honey to boil, being sure the combination is thoroughly dissolved. Pour the boiling syrup over the greens, then swiftly bring once again to a boil. Pour into sterilized jars and seal if desired. SANDWICH SUGGESTIONS

Purslane, peanut buttter and mayonnaise on sprouted wheat.

Purslane, cream cheese and lox on rye.

Purslane, bacon and tomato with mayonnaise on wheat toast.

Purslane and tomato topped with grated cheese, broiled, served with mustard.

Purslane and cucumber with butter or mayonnaise, salt and pepper on thin white bread.