An Introduction to Nature-study

by E. Stenhouse

Fig. 129.—The Horse Chestnut.

The horse chestnut.—The horse chestnut differs from all our other forest trees of equal size in bearing brilliantly coloured and conspicuous flowers. The flowers are complete, containing not only stamens and pistil, but also a calyx and a beautiful pink or white corolla. The tree has a striking appearance at all seasons of the year. In winter, the thick twigs, the large terminal buds, and the opposite lateral buds already described on [pp. 62-64] are very conspicuous, and give an instructive clue to the method of branching. The buds open early and the new shoots rapidly lengthen, so that the tree is a mass of foliage whilst most neighbouring trees are still bare. The leaves ([Fig. 26]) are compound, consisting of seven large, spoon-shaped leaflets which spring from a common point at the end of the leaf-stalk. The flowers open in May. The fruit is ripe in October and then falls to the ground, its prickly husk splitting into three parts to liberate the rounded seeds. The leaves fall early, leaving large scars ([Fig. 38]), which have some resemblance to the hoof-marks of a horse.

34. CONE-BEARING TREES.

1. The Scotch pine.—Notice the shape of the tree: the tall straight stem and rugged bark, and the dark tufts of foliage. Does the tree bear leaves all the year round? Does it ever shed its leaves? Are they shed at any special time of the year? Examine a leafy twig; observe that the leaves are needle-shaped and come off in pairs. At the apex is a terminal bud which will continue the length of the twig next season; the lateral buds below will grow out into twigs at the same time. Try to make out which parts of the twig grew during last year, and which during the two previous years.

Notice the cones. The young female cones (b, [Fig. 130], 1) are erect, and their scales separate slightly in spring to allow the pollen to enter. Afterwards they hang down (c) whilst the seeds are ripening. Three years after pollination the scales come apart again to let the seeds fall out. Examine the seeds from a ripe cone and notice the attached wings ([Fig. 130], 4). Examine cones of various ages. In spring look for the pointed cones of male flowers ([Fig. 130], 1, a) which produce the abundant pollen.

2. The spruce fir.—Compare the spruce fir. Notice the conical Christmas tree shape and the large spreading branches near the ground. Compare the leaves and cones with those of the pine.

3. The larch.—Compare the larch with the pine and spruce. Notice the drooping boughs, the alternate tufts of leaves, and the small cones arranged in a row along the twig. Is the larch an evergreen?

Cone-bearing trees.—The cone-bearing trees, such as the pines and firs, are true flowering plants, but of a type which is very different from any hitherto described. The flowers are peculiar, and form cones, the male flowers producing pollen and the female flowers ovules. The ovules, however, are not enclosed in ovaries, but are naked, so that the pollen gains access to the ovule directly, and is not received on a stigma. The female cone consists very largely of smooth scales, a pair of ovules being borne by each scale near its base. The pollen grains of these trees are rendered particularly buoyant by being blown out at the sides into little air-filled bladders, and are thus easily carried by the wind. When the pollen falls on the female cone, the grains slide down the smooth scales and very likely come in contact with the ovules at the bottom. Each ovule has a sticky drop of gum at its end, and the pollen is caught in the gum. Such pollen grains as roll off the upper scales are almost certain to fall on a lower one and reach the ovules.

The Scotch pine.—This is a large tree, with a dome-shaped crown of foliage. The bark is rough and scaly. Its foliage leaves ([Fig. 130]) are long and needle-shaped, and occur in pairs, each pair being carried by a very short branch. The leaves do not fall off each winter, as do those of most forest trees, but remain on the branches for three years or more, so that the younger twigs are clothed with foliage at all seasons. The cones of male flowers ([Fig. 130], 1, a) are found at the base of some of the shoots of the current year. The female cones ([Fig. 130], 1, b) are formed round the ends of the young twigs. They consist mainly of overlapping woody scales, each being knobbed on its exposed surface and bearing a couple of ovules near the base, where it springs from the axis of the cone. The young cones are erect, and in spring (when clouds of pollen are blowing about) their scales separate slightly to admit the pollen in the chinks between them. After pollination the scales close again and the cone hangs down ([Fig. 130], 1, c). The ovules receive the pollen in May, but the actual union, or fertilisation, does not take place until June of the following year. The seeds become mature two years after fertilisation. When they are ripe each bears a thin wing which has split off from the upper surface of the scale. The scales now separate, and the winged seeds fall out, to be distributed by the wind. When all the seeds have fallen, the empty cones drop off the tree.