Watching To Have and Have Not the other night, shortly after the news that an American tobacco company had conceded that cigarettes were addictive and harmful, turned out to be a strange experience. Even at the best of times Howard Hawks' film has problems with its plot, a desultory Casablanca rip-off which serves principally to separate Bogart and Bacall's smouldering scenes together - much as the graphite in a nuclear reactor isolates the fuel so that the whole thing doesn't go critical. But to watch the film with that news item in mind was to be made aware of how literal the description "smouldering" is. Barely a scene in the film is without a wreath of smoke rising from the glowing tip of a cigarette. Indeed, when Bogart finally takes his boat out on its hazardous mission it's perfectly conceivable that it is not a fog-bank he travels through but a fag-bank, a dense cloud of blue smoke that has drifted out from the relentless fumeurs of Martinique.

The experience prompts some thoughts about the vulnerability of cinema as an art form, in particular its dependency on a real world as a source of its imagery (not the real world, which is an altogether more abstract construction). Because of this, films are unusually prone to the unpredictable effects of ageing. If a crime novel refers to a car, for example, we are unlikely to construct the vehicle in all its exactness as we read. What matters in the story is likely to be its relevant "carness" - whether that is a question of convenience or luxury or speed - rather than whether it is a 1947 Packard or a `43 Cadillac. On film, however, we have no choice about such matters - and a vehicle that at the date of production may have been rich in implications inexorably decays to the status of mere period detail. This is true of all forms of realism, of course, but the cinema image is so permeable, so drenched with extraneous detail, that it is far more susceptible than other fictions to perishing.

The flip side of this dependency is that film must - to some degree at least - acknowledge the real world's obsessions, a fact which may actually result in bizarre distortions. Barely anyone smokes in the current cinema, for example, which is hardly true to the life we know, but does reflect the relatively recent status of cigarettes as objects of anxiety. And in that respect, To Have and Have Not reminds us just how much poetry the cinema has lost in being obliged to give up smoking. It is an anthology of the sensual possibilities of the cigarette, from the veiling beauty of a smoke-filled room to the intimacies of a mutual desire. "I need a match," says Lauren Bacall, materialising against the door of Bogart's room, and you know she has finally met him - a man who smokes as much as she does.

There are incidental beauties here - the way in which a match can be used to light up an actor's face, the temporary gauze of smoke drifting across the eyes - but more importantly, there is a wonderful exploitation of the cigarette's talent for innuendo. Sitting across from Bogart, Bacall sighs out a perfect plume of smoke, a spectral limb which reaches up to caress his face. More audaciously, there is the scene in which the cafe is raked with gunfire and Bacall is knocked to the floor by Bogart. "I think I'm sitting on somebody's cigarette," Bacall murmers throatily, squirming sideways to retrieve the butt beneath her butt. The implications of tenderness, heat and pressure make the scene go woozy with sexual promise.

This can easily get out of hand, it's true. In Cigarettes are Sublime, Richard Klein's alternately intriguing and infuriating book about the philosophy and culture of smoking, there are some very funny examples of Freudian over-excitement at the sight of a snout. Klein's own speculations about Casablanca are somewhat undermined by the fact that he starts from an assumption about "the taboo in 1942 that prevented Hollywood films from showing women smoking", a breathtaking disregard for inconvenient facts. If that's true, which damn fool forgot to tell Bette Davis, not to mention a hundred other feisty dames? And while Klein properly reminds us that a cigarette is sometimes just a cigarette, he's inclined to get a little dizzy-headed himself about their cerebral content: "Cigarette smoke is one of the material substances in the world that closely resemble the substance of thought", he writes elsewhere. Some of Klein's thoughts are hazy and some of them make you choke, it's true, but I'm not sure that's what he had in mind.

This sort of writing makes you grateful for how much Klein leaves out about cigarettes in the movies. What would he have made, for instance, of the tender scene in Rio Bravo where John Wayne's sheriff, seeing his alcoholic deputy (Dean Martin) struggle to make a roll-up with trembling hands, intervenes to finish the task? You can imagine a paid-up Freudian almost fainting with interpretive joy at this point - Don't you see? Wayne, the figure of virility, restores the integrity of Martin's useless phallus.

But even if you don't share the peculiar self-congratulation of such criticism (the way it pats itself on the back for exposing the "innocence" of our predecessors) the scene remains a brilliant piece of investment on Hawks' part - it is about a kind of impotence and about what a steady hand can do for a man. In their universality and in their private etiquette of need (a smoker never needs an introduction to another smoker), cigarettes offered the perfect medium for such emotional transactions. Whatever we think of the habit, we should mourn the fact that Hollywood has kicked it