Holly isn't just for Christmas - it's winter's shining star

Scientists say fields of shiny plants could reduce maximum temperatures by nearly 2C by reflecting the sun's heat back into space. Perhaps we're all becoming a little weary of things that scientists say, and those that think they are saving the world are particularly shrill in their saying.

But plant breeders must believe there is something in the theory since they have developed a strain of soya bean with leaves that reflect 5 per cent more sunlight than other types. That means it can be grown in places hotter than it would normally accept. And usurp a plant that over millions of years has by its own means adapted to the conditions that the soya, left to its own devices, would reject.

And you can bet that those fields of shininess would not be planted up with beautiful things such as holly. Too slow. There would be acres of hideous rubber plants, sheets of philodendron. But in the phenomenal bright, cold days that linked Christmas and the New Year, the thing that most struck me about holly was not its berries (all eaten by November), nor its prickles, but its glitter. Though dark-leaved, it is by far the brightest tree in the garden, reflecting the low winter sun in all directions.

Holly, one of our rather small collection of native evergreens, carries plenty of symbolic baggage along with its gloss. It stands for life in the bleak midwinter and, in country areas at least, still commands great respect. Even mechanical hedge cutters, which respect very few things, are guided carefully round a holly's smooth-skinned trunk. The dark, lustrous beacons rise up from lane hedges more often than any other tree.

How many millions of years did it take for holly to design its defence system? You can see why these evergreens need them. As winter strips the landscape bare of leaves, evergreens become magnets for browsers. Yew concocted a deadly poison; ivy, which sheep graze voraciously in winter, got itself off the ground and into the air; holly has its prickles.

The clever thing about the holly's prickles is that they work in so many planes. Leaves generally are flat, two dimensional, like stencils. The holly leaf has a central midrib, like any other leaf, and up to eight pairs of veins branching out from the midrib in parallel lines to finish in the spines on either side of the leaf. But then symmetry is abandoned and every point takes its own line. Some curl back under the leaf, some turn up to the sky, some fill the slanting voids in between. It means that whichever angle you approach it from, the leaf will jab you. And shine at you.

The way the points turn must have to do with the relative rate of growth of the cells on the upper and the under side of the leaf. If the cells grow faster on the top, the point will be pushed down. If there is manic activity on the underside, the point will be forced up. But it's strange how each point knows what its neighbour is up to and makes sure to do something different. Defences, as medieval fortress builders quickly worked out, are much less effective if all points turn in the same direction.

Think of the leaf as a rugby forward line, said a botanist friend. If a winger, or next door point, darts off in a certain direction, the rest don't follow, but do what will be best for the team, covering the gaps where the winger isn't. Rugby players use their eyes to fix where they ought to be; leaves react to activity in neighbouring cells.

And yes, I know that a football analogy would be much more fashionable, but being Welsh, I was 22 before I ever saw a football game. It was at Liverpool and they were playing Everton, but even that iconic match seemed to have little of the drama of the Newport/Cardiff rugby jousts on which I had been brought up. I was shouting for Everton, which, given the venue, was perhaps unwise.

But the botanist couldn't tell me why they shine so, those holly leaves. Nor why ivy leaves, quite a matt, dark green when they are on the ground, or climbing flat against a wall or tree, become brilliantly shiny when they develop into their own shrubby mounds: arborescent ivy, it's called and it's the stuff that berries so beautifully in winter. The leaves change shape then too. Instead of being three-fingered and generally triangular, they become much more rounded. The gloss at this stage is very good, though not quite so eye-catching as the holly's. The ivy is a slightly paler shade of green. Perhaps that is why the light seems not to reflect so dramatically from it. And it doesn't have the holly's points, which twist each leaf to reflect light from whichever direction you look at it.

Camellia (ours very fat with bud) has gloss. So has laurel and fatsia. None, though, shine quite like holly. In the deepcut tracks around our place, hart's tongue fern is the king of glitter, not evergreen like the holly, but winter-green. You can already see the tight fists of this year's new fronds clenched on the ground, the long strap leaves of last year tough as leather, but smooth, lustrous, catoptric. .........

So, if we believe, like the scientists of the American Geophysical Union, that glitter matters, what else should we be planting in our gardens to reflect the heat of the sun? Or to liven up our winters? Unfortunately, not all that glitters is hardy. Aeoniums are fabulously glossy, but ours have to spend the winters tucked into a cold frame as they would melt into mush if exposed to the kind of frosts we've had over the last couple of months. But phillyrea, which you can clip into cones or balls, is quietly burnished. Magnolia grandiflora glistens very beamishly. Star jasmine (Trachelospermum jasminoides) glows modestly. It is not fully hardy, but a sunny, protected wall is all it needs to survive through winter. If you're really worried, you can carpet the terrace or deck with leftover mags. Vogue and Harpers should do the trick.