Alfalfa and mung beans are just the beginning for the sprout enthusiast

There are people living in apartments high above Manhattan's crowded streets who boast no balcony and even lack an appropriately sited windowsill. Yet they all grow significant quantities of food indoors all year long.

It is a practice, in fact, that is being duplicated in towns and cities all over the country. Even people in rural settings, with all the outdoor gardening space they could want or ever need, are doing the same thing.

Simply, they are growing sprouts.

To be more precise, they are sprouting seeds (hence the term sprouts) which are eaten fresh, cooked, or in baked form within a few days of germination.

The beauty of sprouts is that they can be grown anywhere - on the back seat of your car if you are traveling any distance, or even in your backpack when you go camping. They can be grown in the kitchen, the bathroom, behind the living-room sofa - in other words, wherever you find it convenient. Moreover, you can grow sprouts in just about anything that will hold water - a Zip-Loc sandwich bag, for instance; a bowl with a saucer on top; or a glass jar covered with cheese cloth or a piece of nylon stocking.

Finally, sprouts are inexpensive and even downright cheap by some standards. While you are unlikely to want an entire meal of sprouts, they can readily form a major part of a meal for just a few pennies a serving. According to Fred Rohe, writing in ''The Complete Book of Natural Foods,'' (Shambhala Publications Inc.) , 1 pound of dried seeds produces 8 pounds of sprouts. A single pound of sprouts is enough for eight servings at a cost of 2 to 8 cents a serving, depending on the price of the seed.

Sprouts need moisture and moderate temperatures, which most people can readily provide. They don't need light and they don't need soil, both of which are not so readily available in apartments. But the reasons for their popularity don't end there. Sprouts provide the quickest of all harvests. You'll be eating them within three to six days of sowing the seeds.

When seeds sprout, they quickly expand up to eight times their original volume and increase similarly in weight. A spoonful of seed readily becomes a cupful of edibles. And the nutritional value of the sprouts increases even more dramatically.

A decade ago few people knew about sprouts and still fewer ate any, other than at a Chinese restaurant. Now everybody knows about them, and so many have made them a regular part of their diet that the supermarket industry has taken notice. Many markets now sell packaged sprouts, principally mung beans and alfalfa.

The home sprout grower can enjoy a far wider choice than that found in a supermarket. Alfalfa and mung beans are just the beginning. Lentils, garbanzos, wheat, barley, soybeans, peas, beans, and oats are readily available as whole grains - and therefore easily grown at home.

By the way, always buy your seeds or grains from a food store. Those destined for agriculture have frequently been dusted with a fungicide to prevent them from rotting in the soil.

Here is the simplest way to grow sprouts:

* Fill a quart jar (preferably with a wide mouth) with warm water and add a cupful of one of the larger grains or three to four tablespoons of smaller seeds. Let the seeds soak overnight.

* Drain off the water the following morning. You can add the water to soup stock, as some of the flavor and nutrition will have passed from the soaking seeds into the water. You can also use the water for your houseplants.

* Place the jar in a convenient spot. This can be in the dark or light (it doesn't much matter), but avoid placing it in direct sunlight. The sprouts will also do best in an area where there is good ventilation.

* Be prepared to rinse the sprouts twice a day. Some growers recommend three rinsings: before leaving for work in the morning, on returning home in the afternoon, and before retiring in the evening. I have always found two rinsings satisfactory.

Rinsing the seeds can be made a simple chore by covering the mouth of the jar with a piece of cheese cloth or nylon stocking held in place by a rubber band. This allows you to tip the jar and let the water drain without fear of losing seeds.

Be sure the sprouts never dry out. At the same time, never leave water standing in the bottom of the jar, or the sprouts may begin to rot.

Mung beans and oats are ready for the table in 2 to 3 days, while most other seeds take 3 to 4 days. After they have reached the length you prefer, refrigerate to stop further growth. Wheat, as tender as any other sprout up to about four days, will quickly develop a fibrous chewiness if left in the jar too long.

You will need to experiment a little and find which sprouts are most popular with you and your family. You will prefer eating some raw and others cooked. All of them can be eaten either way. Stir-frying is a great way to go, and they ''belong'' in any soup or stew. Innovative bakers add them to breads. ''The Natural Foods Epicure'' by Nancy Albright (Rodale Press) lists a number of recipes containing sprouts, including a wheat-sprout candy.

A number of commercially made ''sprouters'' are on the market and are generally sold at natural-food stores, but the wide-mouth jar works just as well. Or you can do as Helen Nearing of Harborside, Maine, has done for a long time. She uses a china bowl with a saucer on top. The saucer keeps the sprouts from drying out and prevents loss during rinses.