From an extract published here
Leaves must produce food out of thin air, or else there is no tree. Luckily for nature and all of us, they are extraordinarily good at it. There is, for example, a stupendously beautiful oak tree at Ariundle, within the Sunart Oakwoods of coastal Argyll, that is perhaps eighty feet tall and of a still mightier girth of limbs. It is also an old acquaintance of mine. Consider first that the whole edifice is the work of its leaves, and that no leaf lives longer than six months. Then marvel at nature. Then believe in magic.
Leaves begin life tight-packed in a bud. In spring, they start to expand, then they start to draw the sap up through the tree. How do they do that? That is absolutely my favourite tree question. Because the answer is that no one knows. We can split the atom and fly to the moon and find water on Mars but we don’t know how a leaf drags a tree up into the air. I find that profoundly reassuring.
The containing scales of the bud respond to the pressure from within and hinge backwards allowing the leaves to open, at which point they go to work, which is food-shopping. Look again at the eighty-feet-high oak tree and take a wild guess at how many tons of timber it holds aloft in a crazy fan shape of idolatrous sun-worship.
Almost all of it, perhaps as much as ninety-five per cent – the fabulous girth of the trunk and almost every bough, limb, branch, twig and twiglet – is nothing more than carbohydrates ensnared from the air by leaves. Before any one leaf is even half-grown, it has stored up more sustenance than it will need for the rest of its life, but it goes on food-shopping because that is what leaves are born to do, and it donates everything else throughout its life to the tree.