Plants that flower in the dead of winter speak directly to our sense of daring. Some, perhaps, produce blooms of a rather tough texture, as if trying to physically outface the worst weather. Hellibores are like this - the favourite in gardens being the white ''Christmas Rose.'' Though admirable, their flowers are not quite what a flower should be; there is too much of the feel of leaves and stalks about them and not enough of that peculiar delicacy which one associates with petals. Other winter flowers are survivors, muffled against the cold by avoiding vulnerable flamboyance, by preserving a dull greenness or by staying as insignificant as possible.
But there are plants, and shrubs, which produce midwinter blossom with tissue as frail in appearance, as fine and intricate in detail, as flowers that belong to an inviting summer warmth. They amaze because it is hard to believe that such tenderness is also proof against bitter frost and cruel sleet. Their self-protection is no outwardly obvious kind of armour.
The snow drop is a plant of this sort. The witch hazel, a tree, but rarely reaching much size, is another. Both have a delicacy in their flowering which defies the shout and bluster of the dark months. They could hardly be more different in appearance, but both know precisely what their time and place is, and in neither case is their winter boldness an unwise temerity.
The yellow spider flowers of the witch hazel are strange and unruly, with petals like minute ribbon ends gathered haphazardly in knots on the thick twigs. Encountered in the bleakness and damp of a woodland at the tail end of the year, they can be a sudden delight. But they are not leftovers from the autumn like the occasional berries found still hanging on some trees, forlorn stragglers missed by hungry birds. And the flowers of the witch hazel aren't premature or overeager: they appear, without trembling, on cue. Certain types of prunus and viburnum decorate their branches in winter with tiny pink flowers, but they generally seem to be there as a false start. A slight improvement in the weather and out they come - but the winter freeze has weeks still to run and returns with force, putting such flowers into a sad predicament.
The snow drop, like the witch hazel, belongs intrepidly to the otherwise dormant woodland in the part of the year that is truly neither end nor beginning. They are not flowers of recollection or promise, echoes or heralds. They are surprisingly solid facts. I have seen bunched and overcrowded colonies of snow drops pushing up to flower under leafless hedges where there is nothing for company but dead twigs and the skeletons of last year's brambles. They can be long established in the most unlikely places. One of their oddest habitats near my house in Yorkshire was a chicken-run. The black earth was scratched bare all round them, but, ignored by the busy fowls, there they were like small explosions of vitality, oblivious, self-possessed, pert, with their dense accumulations of silver-green leaf spears, and, pendulous, on thin stems, the small white flowers. Closely scrutinized they are a marvel of elegant design: three outer petals, opening like insect wings to reveal three inner petals, delineated by bright green markings which make daintiness seem a virtue. They are flower form at its purest and most exquisite. The words of Sir Thomas Hanmer's Garden Book of 1659 are sensitive to their charm: they are plants ''whose pretty pure white bellflowers are tipt with a fine greene, and hang down their heads.''
Till now I believe I have unthoughtfully considered snow drops a sign of spring. But looking again at this painting I am not so sure. Winifred Nicholson told me once that spring was her time of year. She made many paintings celebrating the small bright stirrings of spring - miniature garden irises, daffodils brilliant against sunlit snow, hyacinths potted on windowsills before distances immeasurably hazed in frosty air.
But in her painting ''Snow Drops and Witch Hazel'' it is winter she observes, and winter's flowers: the brave energy of plants alive when everything else seems lost in spectral sleep and vague dreams.
She paints both plants for their singular, simplified character: for their unpretentiousness. They give no more than is necessary for their quality to be seen. They are painted with aptly tuned understatement. Nothing could be further from the art of this picture than the rush and flourish of full-blown summer.
Winifred Nicholson took friendly exception, one time, to something I had written about her paintings. ''I find,'' she said, ''that it conveys Pretentiousness.'' This was the ultimate distortion, I felt, of her work. I had written as though her pictures were ''masterpieces,'' rather like praising the cool, neat felicity of the snow drop in language suited to the peony.
My difficulty was to express in words a painter's intangible sensitivity, apparently casual but actually precise, gentle but knife-sharp. Here was a vision as strong as a winter flower, and as unexaggerated. ''We both know,'' she wrote, ''that the chirps of sparrows are every bit as valuable as the songs of nightingales, as the music of the birds of Paradise. But let's not call our chicks . . . anything but what they are - glimpses at best.''
Her picture ''Snow Drops and Witch Hazel'' is - in flower form and paint form - just such a glimpse. It eludes my words.