Tiny flecks of dust, nibs of paint or protruding knots in the weave of a canvas catch your eye as you follow the smooth, unerring lines of Agnes Martin across her painterly plains of colour. Something of an outsider, Martin grew up in Saskatchewan, Canada and only graduated from art school aged 40. She then retreated to Taos, New Mexico to paint and teach art, although Georgia O'Keefe, Mark Rothko and others had already found spiritual homes in the area.

The temptation is still to see aerial landscapes, parched fields and horizons in her early works, titled 'Beach' (1958) or 'Harbor' (1957). Yet anything resembling a tree or a sunset was very quickly replaced by simple grids of hand-ruled pencil lines and bands of pastel-pale reds, blues and yellows. There are still traces of never-ending fields and powerlines, earthy tans, taupes, khakis and ochres, but Martin went on to claim that her contemplative works were actually 'anti-nature'. Indeed, later pictures in Tate Modern's superb retrospective resemble nothing more than grey concrete slabs or grids of skyscraping windowpanes.

Martin was 'discovered' by a gallery owner in 1957 and encouraged to move to New York, where she straddled the still dominant school of Abstract Expressionism (she was the same age as Jackson Pollock) and the early seedlings of Minimalism, another boys-only club that she felt even less kinship with. In 1967 she quit the art world for an extended road-trip, ending up back in the desert in an adobe mud hut she built for herself.  But making claims of unadulterated artistic naivety or isolated genius on her behalf just don't wash, if you will excuse the pun. Neither are these tasteful, abstract wall-fillers (although her prices are through the roof).

Martin was skilled, calculating - destroying many works she deemed below par - and often bold in her painterly constructions, as can be seen in the magnificent, crowning room of The Islands I-XII (1979), an immense series of blindingly brilliant, off-white harmonies stretching off into infinity. Elegant and tough, sure, but never effortless or easy on the eye, Martin's soft-edge abstractions and too-watery paintings still seem fresh 50 years on.

TAGS: LONDON, TATE MODERN