In autumn these leaves change colour: they become a pale brown, and will hang for weeks rustling in the branches till the young buds which are to appear next year begin to form and so push the old leaves off. If a shrivelling frost or a blighting insect destroys all the young Oak leaves, as sometimes happens, then the sturdy tree will reclothe itself in a new dress of leaves, which neither the Beech, nor the Chestnut, nor the Maple, could do. It shows what a great deal of life there is in the stout tree.
The flowers of the Oak arrive about the same time as the leaves, and they grow in catkins which are of two kinds. You will find a slender hanging catkin (3) on which grow small bunches of yellow-headed stamens (4). Among the stamens you can see six or eight narrow sepals, but these stamens have no scales to protect them as the Hazel and Birch catkins have. On the same branch grows a stouter, upright catkin, and on it are one or maybe two or three tiny cups (5), made of soft green leaves called bracts, and in the centre of this cup sits the seed-vessel, crowned with three blunt points. As the summer advances this seed-vessel grows larger and fatter and becomes a fruit (6) called an acorn, which is a pale yellow colour at first, and later is a dark olive brown. The soft leafy cup hardens till it is firm as wood, and in it the acorn sits fast till it is ripe. It then falls from the cup and is greedily eaten by the squirrels and dormice, as it was in the olden times by the pigs. From those acorns that are left lying on the ground all winter, under the withered leaves, will grow the tiny shoots of a new tree when the spring sunshine comes again.
The Oak tree is the most hospitable of trees: it is said that eleven hundred insects make their home in its kindly shelter. There are five kinds of houses, which are called galls, built by insects, and you can easily recognise these, and must look for them on the Oak tree. Sometimes on the hanging stamen catkins you will find little balls like currants with the catkin stem running through the centre. These are the homes of a tiny grub which is living inside the currant ball, and which will eat its way out as soon as it is ready to unfold its wings and fly.
Often at the end of an Oak twig you find a soft, spongy ball which is called an Oak apple. It is pinkish brown on the outside and is not very regular in shape. This ball is divided inside into several cells, and in each cell there lives a grub which will also become a fly before summer is over.
Sometimes if you look at the back of an Oak leaf you will see it covered with small red spangles which are fringed and hairy. These spangles each contain a small insect, and they cling to their spangled homes long after the leaves have fallen to the ground.
Another insect home or gall grows in the leaves, and this one is much larger, sometimes as big as a marble. It too is made by an insect which is living inside, and this is called a leaf gall.
There is still another insect which attacks the leaf buds and causes them to grow in a curious way. Instead of opening as usual, the bud proceeds to make layers of narrow-pointed green leaves which it lays tightly one above the other, like the leaves of an artichoke or the scales of a fir cone. If you cut one of these Oak cones in half you will find many small insects inside, which have caused the bud to grow in this strange way.
And there is one other oak gall you must note. When the leaves have all fallen and the twigs are brown and bare, you see clusters of hard brown balls growing on some of them. They are smooth and glossy and the colour of dried walnuts. These also have been made by an insect. Sometimes you see the tiny hole in the ball by which the grub has bored its way out. This kind of gall does great harm to the tree, as it uses up the sap that should nourish the young twigs.
The wood of the Oak is very valuable. Sometimes a fine old tree will be sold for four hundred pounds, and every part of it can be used. The bark is valuable because it contains large quantities of an acid which is used in making ink; also in dyeing leather. Oak that has been lying for years in a peat bog, where there is much iron in the water, is perfectly black when dug out, black as ink, because the acid and the iron together have made the inky colour.