Keep off the grass

What should we value the early Gainsborough for? His extreme precocity? His "charm"? His freshness and fluency with the brush? His love of the English countryside? (Certainly not for developing into the later Gainsborough.) Perhaps just for the sake of a single picture which is more than all those things - and really the only point to the National Gallery's small and far from comprehensive Young Gainsborough exhibition is to show, by comparison, how remarkable that picture is.

As for Gainsborough's precocity, you need only look at the dates, though they're usually approximate. The first pictures here come from his late teens; the last, from his early thirties. He seems to have known what he was good at from the start. The earliest item, Bumper (a hound portrait in a wood), shows Gainsborough putting his hand to effects - sudden, unpredictable lights, startlingly pure colours - which he refined, but didn't essentially alter for many years.

Effects, they are. Gainsborough may have taken notes al fresco, but his naturalism is a hit-and-run affair, a matter of telling details. His abiding and often expressed "inclination to landskip" reflects less a native affection for the Suffolk country, more the opportunity it provided for a good free paint. The Wooded Landscape, for instance, is pretty well a fantasia of billowing autumn foliage, and don't enquire too closely into the species of the trees involved.

Pure landscapes, it happens, are rather outnumbered in this show by outdoor portraits - partly the result of what could be rounded up from British collections. Here Gainsborough is not so fluent. He suffered an occupational hazard: the human figures were painted from dressed manikins, a common practice for the jobbing portraitist, with the sitter's face added at the and, and it shows. These characters have been compared to bottles, which makes them sound sweeter than they usually are (though still preferable to his later portraiture from life, those oddly monstrous figures that always suggest human versions of a pantomime horse). And John Plampin, reclining doll-like on a grassy knoll, is sweet. It's a small picture, and it's as if a little fellow in a bonsai world had been depicted actual size.

The single picture that tops all these is also in this genre. I mean, of course, Mr and Mrs Andrews. It's Gainsborough's best landscape and best portrait. It can't be called the culmination of anything because he painted it when he was about 21. It can't be called early promise because he didn't better it. A great one-off.

The scene is Suffolk, though even here it probably doesn't show a specific view. The general impression is apres-shower loveliness: tailored to the occasion, no doubt, for this is a wedding portrait, and I don't think, at this stage, Gainsborough had what you could call a personal vision of landscape (that came later, and evolved into a tedious moodiness). But there were definite things he liked doing with nature, and in the foreground here you have one of them - the stretch of grass. At least, it's green and it stands for grass, but what is this flowing surface? Meringue? Velvet? Smoke?

In fact it's a quite non-specific substance, pure paint invention, which - just change the colours - would do equally well as earth or sky. Green turf here: but it turns up again (ochre) as the cascade of chalky mud in Cornard Wood and (red-brown) as the clay base of John Plampin's knoll. It's a sensation that Gainsborough evidently delights in: a dream-stuff, both impalpable and resistant, wet and dry, the way children imagine landing on clouds.

And it's perfectly precise. In this picture, everything is. The other outdoor portraits have big focus problems. There's a two-way split. The landscape lapses into vaguely suggestive background, while the figures stand out as crisp and shiny indoor portraits. Mr and Mrs Andrews solves this by picking everything out. It's a kind of mild caricature. It's obvious in the figures, the way the casual droopiness of the man is set off by the very long, straight and thin barrel of his gun, and the woman's enormous dress with its flying hips terminates in her tiny, dainty, pertly crossed feet. But then, an air of caricature touches the natural world, too.

The scene is itemisable. There are actually rather few component pieces, and each one is distinct and pointed. There's a range of registers: the smoky grass, the high finish of the big tree-trunk, the graphic notation of the wheat-sheaves, the fluid sketchiness of the iron bench. Each item keeps its singular character. But at the same time, with the elements all clear, Gainsborough sets up a relay of rhymes and play-offs. There's hardly an object or a shape that doesn't find a relation somewhere else in the picture.

The bench, for example, does a lot of work. Its twisting legs licking the twisting tree-root, its serpentine frame picks up both the hip of the dress and the sinuous tree branch above, its curling arm turns with the slow curve of the field. Little significant comparisons keep striking - the man's stiff tricorn hat as against the woman's floppy bonnet - and you half expect some sort of large moral scheme to emerge from the echoes and contrasts, some male-female line-up of forms perhaps. I can't see that it does. These recurrent felicities are formal business. Which is not to say that the picture doesn't have a subject; and that subject isn't the perfect harmony of man and woman and bountiful nature.

It was Mr and Mrs Andrews that John Berger cited in Ways of Seeing as an example of how landscape painting celebrates private property. "They are landowners and their proprietary attitude towards what surrounds them is visible in their stance and expressions... Among the pleasures their portrait gave to Mr and Mrs Andrews was the pleasure of seeing themselves depicted as landowners..." And to stress the point, in his TV series, a little sign was montaged on to the picture saying "Trespassers Keep Out".

The point is well made. The only problem is that the example is too good for the general argument. Many landscape paintings might serve their landowning commissioners as tokens of possession. But this picture is about property, explicitly so.

Its action is more like an assertive property claim - all ours, right? - than a bland statement of secure tenure. Its main opposition is between the figures and the landscape: a landscape that, pictorially, the figures don't quite own.

It's an unusually wide format for a two-figure outdoor portrait, and evidently the Andrews only occupy half the picture. They're hemmed in to one side, and all the other side is open to the land. They're far from commanding the whole composition: the broad expanse of country puts them on the spot. Or take it from the angle of the "trespassing" viewer. The picture says both "Back off" and "Come in". The Andrews stand guard at the entrance of the picture (so to speak), defending their territory. But design-wise, the scene is all invitation, the receding curve of the field's edge encouraging the viewer's right of way.

So the picture stages a classic property-owner's dilemma. The more you have, the more isolated you are within it, the less you can prevent ingress. I'm not suggesting that Gainsborough was a closet rick-burner, or in any way disputed the Andrews' rights to these rolling fields; only that, out of a standard sort of subject-matter, he made a drama. He never did anything else like it, early or late

`Young Gainsborough': National Gallery, Trafalgar Square, London WC2 (0171-839 3321) to 31 March