Joan Miró, Tate Modern, London

What do you call a man who paints the same thing for 70 years?

If that man is Joan Miró, the choice is wide. "Surrealist" is a hot favourite, although so, confusingly, is "abstractionist". Then again, you might try "internationalist", although Miró was a regional artist. The diehard Catalan saw Freud's subconscious as his natural habitat, although he also saw himself as a political painter of modern Spain. In truth, he was all these things and more, often at the same time. Which might make you think of other words for Miró – opportunistic, say, or inconsistent – although Tate's new show suggests he was not.

Confusing? Yup. Let's start in 1918 with a maize flower. Miró, born in Barcelona in 1893, and then aged 25, painted Vegetable Garden and Donkey at the family homestead, a farm near Tarragona. The image is studiedly unsophisticated. Its cack-handedness and amateur perspective mark it down as bucolic, which is to say that Vegetable Garden and Donkey is having nothing to do with Madrid and Castilian-speaking Spain. But the homeliness of the work also makes it private, inward-looking: Catalonia and the unconscious are the same place for Miró. There, in the middle of the work, is the maize flower we're talking about, painted as a child would draw a star, a burst of intersecting lines.

Keep an eye on that star, for we will see it again. And again. Five years later, it is in a bottom corner of Catalan Landscape (The Hunter), although it has morphed, too, into the bacterial-looking thing in the picture's upper middle. In the pastel Figure (1934), the star is five-pointed and trails a comet-tail; in the charcoal drawing Naked Woman Climbing the Stairs (1937), it is back to being six-pointed, each point in turn having a six-pointed star. In Seated Woman II, its rays are like the flagella of a bacterium, although they can also be read as a vagina with surrounding hair. Thirty years on, in Drop of Water on Rose-Coloured Snow (1968), Miró's star is still in the ascendant, back to being the child's maize flower he had painted 50 years before.

To all of which you might say: so what? Consistency isn't necessarily a good thing when it comes to art, yet this artist was always the same differently. More so even than his hero and countryman, Picasso, Miró found his vocabulary early on; invented it, really. The meanings of his words might change – star as bug, star as maize flower, star as star – and so might the syntax that holds them together: Vegetable Garden and Donkey and Drop of Water on Rose-Coloured Snow are vastly unalike as pictures. But they are joined by a sense of wonderment which Miró kept alive for seven decades.

I say this, and yet I'm not really a fan. This may, in part, be because Miró's symbols – the blobs and tendrils, the cut-out fish, bacilli, ladders, stars – feel whimsical. Miró seemed to have spent 70 years playing in a Jungian sandbox, and there are few things less attractive than a 90-year-old child. But the problem with a lot of Surrealist art (let us for a moment call Miró a Surrealist) is that the subconscious mind, being quick and inchoate, isn't really amenable to being painted. The Tate does Miró a great favour in showing him to have been a steelier artist than one might have thought. If he is often seen as having influenced American Abstract Expressionism, then the room containing Miró's two vast triptychs from the early 1960s, painted after a trip to New York, shows that the reverse was at least as true.

And then there is the remarkable year when it all comes together, when Miró is Miró-esque as never before or again. In 1940-41, caught by the war in a Norman seaside town, he painted a series of works known as The Constellations; the Tate gives nine of them a room of their own. Faced with a choice between life under the Nazis or under Franco, Miró, grudgingly, returned to Spain. The Constellations are a summing-up, as he thought, of what he was and what he had become.

Small and on paper, they depict a world with the accuracy of scientist looking down a microscope, a place of microbial things swimming densely. For a moment in a career that was too long and in an art that was too free, Miró does what he had always wanted to do. He describes a place that is real and concrete.

To 11 September (020-7887 8888)

Next Week:

Charles Darwent dips into Revealed, at Turner Contemporary, Margate

Art Choice

In London, Tate Britain restores the reputation of the Watercolour, with its thorough "history of" exhibition (to 21 Aug). Spanish artist Jaume Plensa comes to Wakefield where his work takes over the Yorkshire Sculpture Park. In this, his first major UK exhibition, the large-scale sculptures can be seen in the five galleries as well as around the park (to 25 Sep).