While it is day the green leaves select from the air a gas called carbonic acid gas. This they separate into two parts called oxygen and carbon. The plant does not need the oxygen as food, so the leaves return it to the air, but they keep the carbon. This carbon becomes mixed in some strange way with the water food drawn from the soil by the roots. Forming a liquid, it finds its way through many small cells and channels to feed the growing leaves and twigs and branches. But, like ourselves, a tree if it is to live and thrive must breathe as well as take food. By night as well as by day the tree requires air for breathing. Scattered over the surface of the leaves, and indeed over the skin of the tree, are many tiny mouths or openings called stomata. It is by these that the tree breathes. It now takes from the air some oxygen, which, you will remember, is the gas that the leaves do not need in making their share of the tree food. Now you can see why it is that a tree cannot thrive if it is planted in a dusty, sooty town. The tiny mouths with which it breathes get filled up, and the tree is half-choked for lack of air. Also the pores of the leaves become clogged, so that the water which is not needed cannot easily escape from them. A heavy shower of rain is a welcome friend to our dusty town trees. As a rule tree flowers are not so noticeable as those which grow in the woods and meadows. Often the ring of gaily-coloured petals which form the corolla is awanting, so are the green or coloured sepals of the calyx, and the flower may consist, as in the Ash tree, of a small seed-vessel standing between two stamens, which have plenty of pollen dust in their fat heads. It is very interesting to notice the various ways in which the tree flowers grow. In some trees the stamens and seed-vessels will be found close together, as in the Ash tree and Elm. Or they may grow on the same branch of a tree; but all the stamens will be grouped together on one stalk and all the seed-vessels close beside it on another stalk, as in the Oak tree. Or the stamen flowers may all be found on one tree without any seed flowers, and on another tree, sometimes a considerable distance away, there will be found nothing but seed flowers. This occurs in the White Poplar or Abele tree. You must never forget that both kinds of flowers are required if the tree is to produce new seed, and many books have been written to point out the wonderful ways in which the wind and the birds and the bees carry the stamen dust to the seed-vessels, which are waiting to receive it.