The original plays that have emerged from the first season of public TV's "Visions" series have enriched television's dramatic fare more at one stroke than a decade's worth of net work gloss and imported chic.

Even if "Visions" had furnished us with only the single example of "The Gardener's Son," written by Cormac McCarthy and conceived, produced and directed by Richard Perace, it would have redeemed every bit of the effort and expense the project has consumed. But the fact is that a goodly number of "visions" entries have proven to be works of conspicuous and enduring merit.

Among the most impressive have been "Liza's pioneer Diary," Nell Cox's warm but clear-eyed account of the westward trek: Conrad Bromberg's "Two Brothers," a penetrating chronicle of emotional crisis and mental stress; "El Corrido," an offbeat Chicano musical that doubles as a morality play; and Jean Shepherd's "The Phantom of the Open Hearth," an affectionately wry trip down mid-America's memory lane. Even those "Visions" entries that proved seriously flawed were far from wasteful flops - Susan Yankowitz "Prison Game," for instance, which stumbles in development but rests on a vastly intriguing dramatic premise.

It cannot be claimed that "The Gardener's Son" is "representative" of the series - there's far too much divergence in subject matter, approach and esthetic worth for that. "The Gardener's Son" may well be the finest "Visions" installment thus far presented, and as a drama on its own its is an accomplishment of the first order.

The play stems from an actual historical incident of the Reconstruction era in rural South Carolina, the mysterious shooting of a cotton-mill owner by a poor young white man, Pearce, the associate producer and cameraman of the Vietnam documentary "Hearts and Minds," was intrigued enough by the case to search out the surviving facts and rumors.

His interest proved contagious when he enlisted McCarthy, a noted Southern novelist, to write a telescript based on the murder. On the surface, the resulting drama is a portrait of the killer, Robert McEvoy, and a study of the submerged animosities between the families of McEvoy and is victim that led to the crime. The story is related with a quiet, measured gravity that makes no attempt to reduce the violent enigma to pat psychological or social causes.

At the same time, the drama illumines many areas of American personal and civic experience - the irreconcilable perspectives of the haves and have-nots; the asphyxiating paternalism of industrial oligarchies; the clash between industry and nature, between the assembly line and individual dignity. All this, however, is filtered through the raw, flinty exterior of the drama, reflecting a writer's accurate ear for local vernacular, and a filmmaker's grasp of the revelatory power of imagery.

The performances by Brad Dourif as the murderer, Jerry Hardin as his father, Anne O'Sullivan as his sister, Nan Martin as the victim's mother and others in a cast of extraordinary mettle, lend the drama an almost Aeschylean austerity and somber depth. A superb original musical score by Charles Gross enshrouds the penetrating twang of Appalachian brass and banjos with a penumbra of tragically cast harmonies.

The production abounds in privileged moments, the impact of which is far greater than the objective details might suggest - in the scene of the hanging, for example, in a mute group of witnesses peers through the cell bars as the condemned is hooded and bound. There is a suspended moment of terrible silence, broken - just above the threshold of audibility - by an awkwardly cleared thothe threshold of audibility - by an awkwardly cleared throat. Yet that one faint, rough sound compresses the whole traumatic pathos of the scene into a single piercing sensation.

By whatever standard one chooses for comparison - live drama of recent times, the imported British products so often hailed as models, or the sporadic efforts of commercial TV - the "Visions" offspring are a cause for pride. The best of them rank with or outshine comparable offerings of the contemporary stage, and in both content and execution they make the British imports - overdraped soap-operas with a tony veneer - look like window dressing for the cultural boutique. What's more, the "Visions" programs are a bargain: most of them, 90 minutes long, cost about $200,000 apiece to produce, on the average; an hour's worth of "Hawaii Five-O," as a not atypical example, weighs in at $385,000.

It is clear that "Visions" has fully justified the initial expenditure of $7 million (from the National Endowment for the Arts, the Ford Foundation and the Corporation for Public Broadcasting) for its first two seasons, including the one just concluded on the air. The mandate: It has brought forth two dozen original plays (14 this past season and 10 more to come in 77-78), expressly composed for television by American authors who have not previously written for the medium. On these grounds alone, the achievement of the series would be hard to overestimate - it would be difficult to find a parallel in TV's artistic past for a creative fallout of this dimension.

Whether "Visions" continues into a third round (as originally planned) during the 78-79 season (the 77-78 productions are either completed or in final processing) depends on financing. The Corporation for Public Broadcasting holds the essential purse strings. Contrary to the impression left by some press reports, CPB has not only given more monetary support to the project than any other single source - a total thus far of nearly $3 million - but has also indicated its renewed confidence in the series this past year by a supplementary grant to ensure completion of the second season.

And according to David Stewart, a CPB staffer recently assigned to keep tabs on matters affecting the series, the "Visions" producers have now been asked to submit specific proposals for additional dramas.

Barbara Schultz, the artistic director of "Visions," reported last week by phone from her Los Angeles headquarters at station KCET that she felt "very much more encouraged recently about the prospects of continuing the series." One can only hope her optimism is justified. It's hard to imagine any better news about television than "Visions" itself, except that there'll be more of it to come.