EW features of the spring attract more attention than the bursting of the buds on our forest trees and shrubs. In fact, the appearance of the little touches of delicate green on the tips and sides of twigs which, for several months, have revealed no outward signs of life, is often regarded as one of the principal indications of the opening of the season. And the general effect of these numerous patches of green, standing out in bold contrast with the sombre background, is so pleasing to the eye, and, withal, so full of promise, that one may well wander by hedgerows and through wood and copse with no other object than to enjoy an open view of the swelling verdure.
It is interesting, too, to note the varied tints of the new foliage of different trees, ranging from a very pale yellowish green to olive and bronze; to watch the rapid progress of the young leaves as they emerge from the grasp of the brown scales which have enclosed them since the previous summer; and to observe the order in which different species respond to the call of spring. But we shall do more than this, for we wish to watch the gradual expanding of individual buds in order to see how the young leaves were folded so compactly in their winter homes; to observe them as they slowly expand; and to see the wonderful provisions made to shield the tender leaves from the dangers to which they are at first exposed.
To carry out such observations we must either pay frequent visits to the budding trees, or adopt some means by which we can watch the expansion of buds at home.
Very frequently we are able to find buds in various stages of development all on the same branch; and when this is the case we have a good opportunity of studying the history of the opening buds by passing our attention from one to another, in proper order. Thus you may often see a small twig of the beech tree bearing buds that show no signs of opening beyond the loosening of the outer brown scales, together with several others in which the young leaves have emerged and commenced to unfold.
But quite a large number of buds will open if the twigs bearing them are placed in water or wet sand, and these may be closely watched at home. Yet twigs selected for this purpose should not be cut too early in the season. Buds must have their natural period of rest; and it is best not to cut twigs for the purpose suggested until the loosening of the scales shows that the buds are ready to expand.
Among the buds that may be successfully treated in this manner we may especially mention the horse-chestnut, beech, sycamore, poplar, and willow. But it must be remembered that there are limits to the growth of young leaves when treated in this way, for they are not supplied with their natural food. Yet the progress they make is quite sufficient for our present purpose, which is to see how the leaves are folded in the bud, and to watch them as they gradually assume their later forms and positions.
It is remarkable that the twigs of some trees, particularly those of poplars and willows, readily form roots when placed in a vessel of water; and these, if supplied with an ordinary mineral fertilizer, or transferred to damp soil, will soon develop into perfect, self-supporting trees; and their leaves will then grow to their full size.
Now let us examine some of the more interesting of the opening buds. Below are some of the beech tree, in different stages. At first the overlapping brown scales which tightly embraced the embryo leaves gradually relax themselves; and, shortly after, a peculiar little mass of closely-folded leaves, covered with silky hair, protrudes at the tip.
When a little farther advanced, we see that the young leaves, which are of a very light and delicate green colour, are folded in a manner that suggests the concertina, or the bellows of a photographic camera, that there is a vein at each projecting angle, and that the whole is protected by a dense covering of tiny hairs. Later still, each leaf widens, so that the veins are slightly separated, and the green tissue is rather more exposed. Even when the leaf has almost assumed its full size it still retains an indication of its former folds, and now we are aware that the silky hairs, which at first formed a complete covering, occupy the edges and the lower sides of the veins only, while the delicate green tissue between is bare and glossy.
Now let us see the reason for this. In the first place we must note that the space within the scales of the leaf-buds, not only of the beech, but of all our trees, is very limited, so that it is necessary for the little leaves within to be compactly folded, crumpled, or rolled. At the same time, the veins of these young leaves are always very prominent and well-formed, while the thin tissue between them is at present only slightly developed. This latter feature is a valuable protection to the leaves; for the thin skin or epidermis covering the blades of the leaves is, as yet, very thin, and not impermeable to water; and if there were a considerable surface of this imperfectly protected tissue, the young leaves would lose much of their moisture and die on dry sunny days. Further, all young leaves retain their folds, crinkles, or scrolls for a time after they have become free; for, in this condition, the thin substance between the veins, still covered with an exceedingly thin epidermis, is less exposed to sun and wind.
In the case of the opening buds of the beech, while the above conditions hold, there is a further protection against drying up afforded by the hairs. We have seen that these hairs exist only on the veins and margins of the young leaves; but at first, when the thin tissue is completely hidden within the folds, and the veins and margins only are exposed, the silky covering is complete. After the leaves have partially expanded, and the outer wall of the epidermis is becoming thicker, the covering of hairs, so necessary at first to shield them from dry winds, is now not so essential; and, later still, when the leaves are fully extended, and sufficiently protected by their perfectly-formed epidermis, the hairs, being no longer required, gradually fall, so that old beech leaves are quite or almost free from them.
The leaves of the hornbeam tree are very like those of the beech, with the same strong, parallel veins; and they are folded in the same manner within the bud, so that the above remarks apply also to them.
If you examine the opening buds of our common forest trees, you will find that in several of them the young leaves are protected from sun and wind by a covering of hairs. Among them we may mention the white poplar, mountain ash, wild pear, and the wayfaring tree—a shrub rather than a tree, very common in the hedgerows of South England, more especially in chalky districts; and, as in the case of the beech, the hairs partially or entirely disappear as the leaves become older. The hairy coat of the last named (the wayfaring tree) reveals a wonderful structure when examined under the microscope; for each hair has several branches all radiating from one point, like the rays of a star, and the branches intermingle so thickly that they form a natural felt.
Some young leaves, not provided with a hairy coat, are protected against loss of moisture by a thin covering of natural varnish that is waterproof; but this, like the hairs, disappears when such protection is no longer necessary.
Again, some newly-exposed leaves, not protected or not sufficiently protected by the means above mentioned, adopt curious devices for the prevention of loss of moisture. One interesting example will be seen in the young leaves of the horse-chestnut. These leaves are compound, each consisting of five or seven leaflets. As they first issue from the bud the leaflets stand erect and close together, thus sheltering one another from the sun.
Then, after becoming so long that this position is no longer possible, the leaflets sink, and hang perpendicularly with their points towards the ground. In this position they do not catch so many of the sun's rays. Finally, when the epidermis is well formed, and the light and heat of the sun become necessary for the functions the leaves have to perform, the leaflets rise and spread themselves horizontally. These precautions appear to be necessary even though the young leaves have a rather dense covering of woolly hair.
Another protective device will be observed in the young foliage of the wild cherry. Here the new leaves are folded only down the middle—along the midrib; and for a time they remain flatly folded in this manner, so that much of their surface is shielded from the sun.
A still more interesting example is afforded by the opening buds of the wayfaring tree already mentioned. When first the leaves appear they stand erect, as is the case with many other species, because in that position they are less exposed to the sun. At this time, too, they are much folded, and the veins are so strongly developed that they touch one another, completely hiding and protecting the deep folds of the green tissue between them. Then the leaves are also arranged in pairs, and are convex on the outer side, so that the margins of each pair fit closely together, forming a closed case round the growing apex of the new shoot.
In addition to all these protective measures, there is the thick, felted coat of hairs already mentioned covering the outer surfaces. As the leaves further develop, and the epidermis is well formed, the veins become farther apart, and the leaves lose their folds and take a horizontal position.
After observing a variety of opening buds we soon come to the conclusion that the so-called "leaf-buds" are really undeveloped branches, for each one eventually gives rise to a complete branch or twig. In many instances the branches bear flowers in addition to leaves, while some buds give flower-clusters only, or flowers with only a few scale-like leaves. We also learn that while some trees produce their leaves before their flowers, others, like the oak, bring forth leaves and flowers at the same time; and others, again, produce their flowers before their leaves, like the hazel, ash, elm, sloe, and some of the willows and poplars.
It may appear strange that some of our trees should produce their flowers so early in the year—often long before the winter is at an end; but there are various reasons why this should be so. In not a few instances the fruits ripen so slowly that the coming winter frosts would destroy them before they were mature if they had not a very early start. Again, the very early flowers that come before the leaves probably have a much better chance of being fertilized in the absence of foliage. If their pollen is distributed by the wind, they are so exposed to the breeze that the process is more likely to be successful; and even if they require the aid of insects it is probable that their prominence compensates for the comparatively small number of insects at present on the wing.