OLD TOWN — The blood-red carrots look like something from Mars, and the tomatoes have unnatural protrusions. The peppers vary widely in shape and hue, while the lemon cucumbers suggest a bizarre mingling of incompatible genes.

Welcome to the gaudy world of heirloom vegetables.

“These vegetables have variations in maturity and appearance,” said Scott Howell, a farm technician at the University of Maine. “That’s not a problem; it’s part of the richness of genetic diversity.”

Howell works at the university’s Rogers Farm on Bennoch Road. He’s a recent graduate of the sustainable agriculture program, which seeks to teach farmers to minimize their purchases of fertilizer, chemicals and other products purchased off the farm.

This summer, Howell led a student garden project at the farm. Undergraduates raised 2 acres of heirloom vegetables to sell at the Orono farmers market and through the Black Bear Food Guild.

Heirloom vegetables are exactly that: old-fashioned varieties that were the staple of American agriculture until the widespread use of hybrid seed earlier in this century.

Heirlooms produce vegetables that are “true to seed.” That means seed from an old-fashioned Brandywine tomato or Oxheart carrot will grow into a plant that is nearly identical to its parents. Pollination occurs naturally and randomly among all the plants in the strain, but years of deliberate selection have ensured that every plant has similar — but not identical — genes.

By contrast, hybrid seeds typically are the first-generation offspring of a deliberate cross between two inbred parents. The resulting seed will grow into highly uniform plants with characteristics desired by the plant breeder — early maturity or disease resistance, for example.

Seed from a hybrid, however, will not produce a vegetable that resembles its parents. In almost all cases, it will be decidedly inferior. That means gardeners who use hybrids must buy seeds every year if they want to continue growing a particular variety — which ensures a ready market for the seed company.

Hybrids are usually more vigorous and productive than open-pollinated strains. But hybridization produces genetic “bottlenecks” that worry Howell and other advocates of diversity.

Hybrids trace their ancestry to just a few parent plants, which can sometimes haunt growers. The most noteworthy example occurred in the early 1970s, when disease wiped out a large part of the hybrid corn crop in the United States because it was all closely related.

According to Howell, it is virtually certain that diseases ultimately will overcome the resistance of the current generation of hybrid plants. Unless a deep pool of genetic material is available to plant breeders, they won’t be able to breed plants that are resistant to the new disease threat.

Heirloom vegetables lack the uniformity in color and shape that gardeners expect when they buy hybrid seeds from major national seed companies such as W. Atlee Burpee & Co. But seed from the larger companies is chosen mostly for its ability to thrive in the moderate weather of the mid-Atlantic states and the lower Midwest.

Conditions in Ohio are very different from the soil and weather in Maine. Heirloom varieties offer Maine growers a chance to adapt vegetables to their own land.

“We have all been too willing to let seed companies take over,” said Howell. “To protect genetic diversity and to have the plants you want, you should grow seeds in your own back yard, adapt them to your own conditions.”

It takes a lot of organization, Howell admits. You have let the plants mature, prevent cross-pollination by different strains of the same vegetable, select the best fruits, dry the seeds, protect them through the winter and nurture them in the spring.

Over several growing seasons, however, you can develop strains that are uniquely suited to your own little patch of heaven.

“The great thing about saving heirloom seeds is that we can look out for ourselves,” said Howell.

And it’s a safe bet that none of your neighbors grows Oxheart carrots.

Heirloom vegetables and flowers are available from several Maine seed companies including: Fedco Seeds, P.O. Box 520, Waterville 04903, or call 873-7333; Pinetree Garden Seeds, Box 300, New Gloucester 04260, or call 926-3400; and Johnny’s Selected Seeds, Foss Hill Road, Albion 04910, or call 437-9294.