I was in a waiting room yesterday, idly leafing though a magazine which detailed the 2006 Henri Rousseau exhibition at the National Gallery of Art. As I looked at the small reproductions of some of his paintings, memories came flooding back of the first time I had met his work.

I was a young girl, working and studying in Paris, and assuaging my homesickness for Africa by spending many hours in the Louvre, the Jeu de Paume, the Musée de Cluny, etc. One day, at the Jeu de Paume, I rounded a corner and came face to face with one of Rousseau's famed Jungle paintings - I am not sure now which one. I was dumbfounded. The painting was so unlike anything else that I was seeing on museum walls; it was seemingly tropical and yet did not ring true at all to me, since I was from the Tropics. The flat, vibrant depiction of these huge, urgent leaves and flowers, growing on strange plants and trees came across as totally hallucinatory. The monkeys swinging from the trees, and other touches of "exotica" were almost perverse in their nuances. The painting left me intrigued.

I learned more in due course about this late-blooming artist, Henri Rousseau, who was born in 1844 and was just ahead in age of another self-taught artist, Paul Gauguin, who embraced the tropics in even more extraordinary fashion. Henri Rousseau had the sobriquet, Le Douanier, added because his main employment, after the military and sundry other activities, was as a minor clerk in the local Customs office. His fellow Customs officials must have been a supportive crowd as apparently they gave him duties which allowed him to devote a lot of time to his art. Despite the recurring theme of tropical vegetation in many paintings, Rousseau never travelled to the tropics: his sources for the plants were the botanical gardens in Paris, especially the Jardin des Plantes. Another large body of work in his very varied opus was paintings of urban-suburban landscapes, complete with chimneys, the Eiffel Tower, streets and tree-lined parks.

Many of these paintings were based on small studies and drawings he did from real life - one of the early plein air painters, in fact. His approach to painting was that of a true Outsider, for he did not follow his contemporaries - Manet, Monet, Cézanne, Seurat, etc. - in their perspective, their realistic depictions, their use of light or even their use of paint. He painted in a flat, decorative fashion, often ignoring traditional perspective, with a Naif optic on subject matter and presentation. Nonetheless, he was eventually recognised as an artist with a great deal of charm and a wonderful imagination, offering a very different version of art. This was despite the long years of derision which greeted the works he submitted to the Salon des Indépendants from 1886 onwards. Henri Toulouse-Lautrec and Edgard Degas began to see in his work a move away from the prevailing naturalism in art, and by the early part of the 20th century, Picasso, Signac and others were showing enthusiasm for his work. Dreamlike worlds, with tigers, serpents, monkeys and buffalo peering though the "jungle", alternate with somewhat airless urban landscapes, portraits, still life studies and other pieces which do indeed prefigure Surrealism. Catalogue images, early photos, books - everything was grist to Rousseau's mill to mix with his everyday observations in these imaginative compositions.

If you want to spend time in a 19th century version of an alternate universe, albeit one which is the product of a fertile imagination allied to a direct vision, then look at Henri Le Douanier Rousseau's art. He led the way for so many later artistic trends - and, most importantly, he believed in himself even in the face of derision and rejection. He just kept on painting, and by the end of his life, in 1910, he knew much success and esteem.