Endless Renovation, Tate Britain, London<br/>Sordid Earth, Roundhouse, London

Shrivelling chrysanthemums, ravishing fuchsias and a single red rose remind us that we all have our moment in the sun

Of all the imagery employed by young British artists over the past 20 years, with the decades' profusion of sex and blood, gutter talk and sliced-up animals, perhaps the most surprising thing has been the return of the flower.

It seems so gentle and subtle a metaphor, reminiscent of the pre-modern world where the otherwise unsayable truths of life and death had to be conveyed through the apparently innocent and exquisite beauty of, say, a bunch of fading roses in a vase.

The intellectuals of Modernism and hipsters of Pop must surely have imagined that the day would never again come for so romantic a symbol, and yet already we have seen Marc Quinn, one of the most high profile of the YBA generation, exhibiting tanks of impossibly perfect blooms preserved for ever in frozen silicone. And now here we have two younger artists taking up flowers as devices for telling age-old stories in fresh and lyrical ways.

In Corin Sworn's installation Endless Renovation at Tate Britain, a vase of shrivelling red chrysanthemums stands on the floor in a pool of bright light thrown by a projector. Behind, the whole arrangement is cast in a bold and forbidding shadow on the wall. The haunting sense of nostalgia is immediate; all the flowers you have ever bought, all the flowers you have ever lost, that sadness we all feel as the petals drop and the inevitable decay sets in. Here, in fact, the vase of flowers acts as a kind of cue to the themes conveyed by the central action of Sworn's piece, in which a pair of projectors rotate through 160 slides selected from a cache of 600 apparently found by the artist in a skip close to her Glasgow home. In a soundtrack to this slideshow, the artist tells how she discovered the pictures and ponders poetically on their origins and possible meanings.

It's a random and rather baffling selection of images: there are American cars and heavily furnished rooms, unidentifiable metal objects and tape measures. But above all there are pictures of flowers, ravishing fuchsias in the fullness of life, dahlias and a single red rose. There are also clocks; not the normal kind of timepiece but Alice in Wonderland-type devices that run from six to six and in which, says the artist, "minutes and hours seem interchangeable".

A small red diary, discovered in the skip with the slides, is also on show. "From this little book I know that our subject had a Citroë* and that on 16 September 1985 he had its tyre repaired," says Sworn. "Yet, I still do not know his name." What she does know, however, is that the person who took the pictures was a clock-maker, as several entries describe the purchase of materials to make the clock cases we see in the pictures.

But why did the clock-maker invent these bizarre machines? A theme gradually begins to emerge as Sworn speculates that these are not clocks of "continuous befores and afters", but instead contraptions that "tell the time of a nostalgic whose afflicted imagination produces erroneous representations of past and present, muddles them".

What we are looking at, I think, is a contemporary contemplation on the themes of the vanitas paintings of the 16th and 17th centuries, with their common symbolism of flowers and clocks to represent the passing of time and fleeting nature of existence. Sworn's narrative, as she muses on the character of her mysterious clock maker, is moving and mournful, a dark kind of fairy tale about a person who tries to cheat time in order to stall the inexorable quickening of life towards its end.

Over at the Roundhouse in Camden, north London, Mat Collishaw is the last in the summer line-up of artists invited to create film installations for a vast circular screen made by the designer Ron Arad. Collishaw is a talented artist of the golden Goldsmiths generation that includes Damien Hirst and Sarah Lucas, but his quiet, contemplative work has often been overshadowed by that of his louder and more shocking contemporaries. This is a shame, as his meditations on the themes of nature and the sublime in art have been consistent and often delicately beautiful in their expression.

In Sordid Earth, Collishaw presents a luscious, 360-degree Avatar-style jungle of trees and waterfalls as a backdrop against which exotic blooms gradually open to full flower. Insects swarm around their giant stamens; their colour is so rich and saturated you can almost smell them. But then things turn nasty as the rot sets in; the petals begin to discolour, and weeping pustules appear before the flowers finally crumple and die. A band of Brazilian drummers mark the passing of time and the whole cycle repeats itself.

As in Corin Sworn's work there is an inbuilt melancholia to all this. The Roundhouse is in a fashionable part of London and at the free evening viewings crowds of bright young things gather in the hollow centre of the huge screen to drink and to chat and to flirt. Many of them seem oblivious to the heartbreaking narrative going on around them. But somehow this only serves to remind the rest of us that however modern we think we are, and however hi-tech or contemporary the methods used to make these memento mori, the tragic truth of their universal and timeless message remains: beauty and youth are transitory – enjoy them and use them well, for tomorrow they are gone for ever.

'Endless Renovation' (020 7887 8888) to 5 Sep; 'Sordid Earth' (0844 482 8008) ends tomorrow

Next Week:

Charles Darwent pulls up at Lee Friedlander: America by Car

Visual Art Choice

American artist Ingrid Calame examines the cracks in the sidewalk, the blobs of chewing gum, and turns them into bright abstract art at the Fruitmarket Garlley, Edinburgh (to 9 Oct). In London, catch those radical, machine-loving Vorticists at Tate Britain before they burn out on 4 September.