Nettle, dock, chickweed, convolvulus, willow herb, thistle and dandelion: there's a lineup of horrors to set the sturdiest horticulturist trembling. They are the impossible weeds, the invaders, the Enemy! And the worst of them - its drinking-straw stems dribbling milk, its juicy, brittle leaves breaking off in the hand to leave a strong branching root ready, with doubled strength, to sprout again, its white parachute seeds carried by the lightest breeze - the weed to cap all weeds is the dandelion.
And yet, for all its troublesome disruption of suburban tidiness and nicety, for all its devastating commonness, the dandelion is still not a ''weed'' as Emerson defined weeds, ''a plant whose virtues have not been discovered.'' They have been discovered. Its roots, ground and roasted, make an acceptable beverage. Its leaves, blanched, can be eaten as salad, like endive. But most of all, its beauty is widely gloried in.
This hated garden weed is one of the bright splendours of wild nature. It is, especially, a child's plant, the exciting find (multiplying uncontrollably along roadsides, colonising building sites, exploding into a fresh blatancy of growth out of tarmac and concrete surfaces), the irrepressible and delightful discovery of the naive eye. To the yet-to-be-taught, the dandelion is no less brilliant a wonder than the rarest lily or the most exotic rose. It is happily innocuous, without thorns or sting, free of malevolence. Each of a million flowers, quite suddenly (it seems) cheering and shouting like a public demonstration, crowding every inch of an unsuspecting field, is a small yellow sun. ''With ample movement,'' wrote the poet Jon Silkin; ''They are a foot high, as you look.'' They arrive, without a hint of shyness, from nowhere. Children grab bunches of this sunlight, but it is a cheat to anyone who wants to own and take it home. As cut flowers, dandelions have no stamina. All they do is close up and droop.
It is as seed heads that they appeal more successfully to the childish need for possessive interaction - to ''tell the time by.'' Blow on the gossamer pincushion and the stars detach and fly, a microcosmos of lightheartedly aimless galaxies, newly created. Easier (and cheaper) than digital calculators, dandelion ''clocks'' have, for children with an understandable disregard for timetables, the inestimable virtue of almost total inaccuracy. Like blowing out candles on a birthday cake, telling the hour by puffing off the seeds of a dandelion reduces the solemnly measured passage of time to a fanciful nonsense. Folk names indicate the popularity of this small pastime: Clock and Watches; Fairy Clocks; Tell-Time; Doon-Head Clock; What O'Clock?
Samuel Palmer, if indeed he is the painter of this unsigned study of dandelions, has viewed it with something of a child's intensity and closeness. He has enlarged it, made it important, concentrated on its disregarded character. He had been attracted to the dandelion in 1825, when his art was at its most particular and vivid pitch, depicting it as a tiny foreground detail in a sepia drawing called ''Early Morning.'' He saw it as a stem topped by a circle of radiating filaments.
The watercolour shown here is freer, and uses colour, and is undoubtedly a later work by an artist who has become less unique in his inspired vision of the natural world. Yet it is marvellously true in form and texture, and still exactly reproduces the minutiae of this enchanted weed. It understands the quickness of the plant, its delicacy in contrast with its coarseness, and it observes accurately the strange squidlike centre from which the light cloud of seeds - now little more than a fluff of white paint - stems.
Palmer was a Romantic of a similar strain to the nineteenth-century English poet Gerard Manley Hopkins, and he celebrated wildflowers in a way he could never have celebrated tame garden ones. Perhaps as a gesture of defiance against the conventions of unimaginative neighbours in Kensington, he deliberately watered and tended weeds in his garden. When a maid helpfully uprooted his harebells, he was not pleased.
He would have undoubtedly concurred with Hopkins' cry: ''Long live the weeds and the wilderness yet,'' and have heartily enjoyed his description of spring, when weeds ''shoot long and lovely and lush.'' Palmer's watercolour says it in the juiciness and gentleness of paint, and the vigour and streak of line: ''Long live the dandelion!'' - a plant overlooked but valued, so utterly commonplace and so utterly miraculous.