Silver Birch

Betula pendula

Three species of birch grow wild in the British Isles and northern Europe. One is the Silver birch, also called the Warty birch because of the little warts on its otherwise smooth twigs; Hairy birch and Dwarf birch are described on pages 21-22. All the birches have white bark, which forms as their trunks and branches expand, and later becomes gnarled and rugged with dark patches. In die Silver birch it is particularly bright, with black diamond-shaped patches. The branches droop at the tips; hence the specific name penduls.

Birch twigs are remarkably thin and whip-like, and carry very small winter buds. The Leaves have an irregular oval or diamond

Silver Birch Winter Buds

figure 16

Silver birch twigs are hairless but bear tiny warts; a male catkin is shown (life size). Below; winter bud (X6).

figure 16

Silver birch twigs are hairless but bear tiny warts; a male catkin is shown (life size). Below; winter bud (X6).

shape and their edges are doubly toothed. Being widely spaced along the twigs, they produce an open, airy crown that lets a great deal of light filter through. Grass, flowering plants, and even young trees can therefore thrive below a birchwood. In the Scottish Highlands many birchwoods are used as pastures and are particularly valuable because they give shelter to sheep from winter storms and snowdrifts. Foresters frequently make use of the light shelter of a thinned-out birchwood, which wards off wind, strong sunshine and hard frosts, in order to establish young crops of shade-bearing trees, such as beech, Norway spruce or Douglas fir.

Germinating Birch Seedlings Botany

figure 17

Birch seedling (xi).

figure 17

Birch seedling (xi).

FIGURE IS

Flowering twigs of Silver birth in April (x^).

Above: male catkins, with adichasium of three flowers (X1G).

Below: female catkins, one being enlarged (X 3).

Birch catkins open in April along with the leaves. The male catkins have the familiar 'lambs-tail' structure of many dichasla set along a long drooping stalk. A single dichasium is shown, much enlarged, in Figure 18: as in the alder, a closely related tree, it consists of three flowers, each made up of a bract, twobracteoles, and three very simple flowers. Each flower has three very small green petals and four stamens, which are forked just below the anthers. After the pollen has been shed the male catkins break up.

Female catkins, shown above at natural size and also enlarged, stand erect and look Uke brisdy green caterpillars. A single dichasium from a catkin of Hairy birch is also illustrated. It is made up of a bract, two bracteoles (hidden behind the flowers) and three female flowers, each having two stigmas. During the summer these female catkins ripen to fruit catkins which eventually hang downwards in a 'lambs-tail' formation that recalls the male catkins in the spring, as shown in Figure It. Each separate flower has now changed into a tiny bruit, holding a single seed, with two wings to carry it through the air. At the same time the larger bracts have become curious three-lobed scales. Both fruits and bract-scales of Hairy birch differ a little from those of Silver birch, as Figure 21 shows.

As the catkins break up in mid-autumn, both scales and seeds drift away on the wind. Seeds that alight on clear damp earth, or in rock crevices, sprout next spring. Nearly every birch tree that you see out in the woods has arisen from self-sown, wind-borne seeds. Birches for growing as ornamental trees are easily raised in a nursery, but foresters seldom plant this hardy tree for its timber.

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