Wolfgang Tillmans, Serpentine Gallery, London

Tillmans' masterclass in botany, the history of art and (a bonus!) ways of seeing

Side by side in the Serpentine Gallery's rotunda hang two shots of what might loosely be called botanical subjects by the German-born photographer Wolfgang Tillmans. Plants apart, the two works could hardly be less alike; plants included, in fact, since one is of a brooding Rhineland forest and the other of a crop of dead ivy growing up the wall of a London council block.

The first, Wald (Reinshagen), is a huge photograph – a landscape in portrait format measuring nearly three metres by two, its titular forest shown soft-grained and creepy by Tillmans' use of black-and-white C-print. The second image, growth, is very much smaller – 40 centimetres by 30 – and is also a chromogenic print, although in colour this time, its palette a bleak confection of red brick and dead leaves. If memory serves (it may not), growth is attached to the wall by Tillmans' trademark two bits of sticky tape; Wald (Reinshagen), by contrast, is conventionally glazed and framed. Seen in isolation, the treatment of the two subjects seems dictated by hierarchy. Forests are serious things, the stuff of Teutonic myth, Caspar David Friedrich and Richard Wagner. They deserve the High Art treatment, big, brooding Expressionism, frames. Dead ivy on British council estates, on the other hand ...

So why have these two images been shown on one wall? A thing to bear in mind as you walk around the Serpentine's 20-year retrospective is that the works in it weren't just made by Tillmans but hung by him. They can certainly be seen in isolation, but they can – must – also be read together. The Turner Prize-winner divides his time between London and Berlin. Wald (Reinshagen) and growth, the one German-titled and epic, the other grungily English and lower-case, suggest two historical ways of seeing. In the context of the show, their differences feel autobiographical, a bi-national stand-off between order and humanity.

Which is to say that the vast array of genres and hangings and hierarchies of image in this show isn't just there to show what a clever boy Wolfgang Tillmans is. Each grouping of photographs makes the point that images do not exist in isolation. We see what we see historically, art-historically, politically, in several ways at once. Tillmans' work makes constant reference to genres of art other than photography. Anders pulling splinter from his foot is a deeply tender and perhaps loving photograph, but it is also a conscious echo of the Spinario, one of the best known pieces of Hellenistic sculpture. Another portrait of Anders shows him as a pebbled Arcimboldo, while William of Orange cuts out the middleman of mere allusion by reproducing wholesale a chunk of Antonis Mor's Kassel portrait of the Dutch king. Three suites of work – Lighter, and Silver Installation VI and VII – are painterly in quite a different way, being assemblages of what look like Minimalist monochromes.

The aim, though, is not to imitate or compete but to trouble. At the heart of all Tillmans' work is a kind of moral doubt – a question mark about what photography is and what it does. The most obvious answer is that it reproduces, by repute exactly and objectively. Lighter 83 is a reproduction in more senses than one. It could be, say, a Donald Judd, and it goes the whole Minimalist nine yards in being framed in a Plexiglas hood. But Lighter 83 is a deeply complex image, its surface being both actually and illusionistically bent, the yellow line that crosses its lower field noticeably ambiguous. Has Tillmans simply made it by exposing his paper to light, or is that yellow line a photograph of something? Is it neutral, or does it have some meaning – some content – of its own?

The real subject of the Serpentine's exhibition isn't Wolfgang Tillmans but the way we see, the way that photography asks us to see. It is an excellent show, one of the best this summer. Go.

To 19 Sep (020-7402 6075), free

Next Week:

Charles Darwent cleans his specs for Close Examination at the National Gallery – a show about fakes (or is it?)