« EelmineJätka »
its fostering oak, are strangers in the land where Virgil sang of the golden bough which formed Æneas's passport into the infernal regions. His bough was undoubtedly one of Loranthus, not of our own true British mistletoe. May we never cease to find it green and bright on our orchard and forest trees, though they be leafless and bare, adding to the delight of our winter walks and our household merrymakings, and may every reader of this little paper henceforth find in a spray of mistletoe a deeper interest, both scientific and historical, than he has ever done before.
Fair Plant, a mystery thy birth,
EXPLANATION OF PLATE VIII.
Fig. 1. Branch of mistletoe, in fruit.
THE WINTER LIFE OF PLANTS.
BY HARLAND COULTAS.
day the field of white snow-drifts which caps the Northern portion of our globe is extending at its circumference. The trees are deprived of their summer foliage, and the race of hardy evergreens appears to advantage. Only here and there a solitary late-blooming flower may be seen, and this at last disappears under the white snow-covering. Nature appears to be devoid of life, as though her pulse had ceased to beat; and we are almost ready to imagine with the poet that the snowy mantle is a pall or winding-sheet thrown over her inanimate and motionless form. Never was there an idea more erroneous. The same formative laws which give to the falling snow-flake its beautiful crystalline forms still operate in the plant-world.
We must, however, except the vegetation within the Arctic circle. The people there never see the sun for months, and without his influence the vegetable machinery will not work. It is, therefore, probable that the Polar plant-world, in all its forms, is in a complete state of torpor and inactivity in winter, during the sun's absence.
But, in lower latitudes, where the sun continues for a few hours above the horizon in winter, his rays are always diffusive of life, however obliquely they fall on the landscape. In all the lower forms of cryptogamic plants there is a considerable amount of active vegetation going forward. Mosses, liverworts, and lichens are very retentive of life, and will grow at very low temperatures-even beneath the snow and ice with which they are covered, provided it is not too deep, so as to prevent the light from getting access to them. Many mosses and liverworts ripen their sporangia or come into fruit in winter. And, as for the lichens, winter is their vegetating period. In summer they are torpid and inactive. Lichens have been very appropriately named by Endlicher, Protophyta,*
* The term Protophyta is now, strictly speaking, applied to the very simplest plants, such as Protocæcus volvor, &c. &c., and not to the Cryptogamia.
or first plants; because they are the very first plants which gain a foothold on the barren rock, and, dying there, prepare the way for a higher vegetation. At this season of the year, when the landscape is no longer decorated with flowers, the moss- and lichen-covered rock has a peculiar interest attached to it, of which it is deprived in summer. There grows the vegetation of winter; and our attention is no longer turned away from these lowly protophytes by the more gorgeous and showy plants which surround them in warm weather.
All the year round there is ceaseless activity in Nature. Plant-life never expires. Even in the depth of winter, there is much organic beauty abroad. Choose your time judiciously, when the sun is up and the weather is clear and frosty, and, if the snow is not on the ground, you will find good botanizing even in winter, if you attend to mosses. You will find these plants almost everywhere. On rocks, fallen trees, on the banks of rivulets, or on the surface of the stones in their pebbly beds; and, if you have a microscope and “ Hooker's British Mosses," where these minute plants are most beautifully figured, you will be at no loss for amusement.
But the active portion of vegetation is not wholly cryptogamous. In the strongest January frost, the trees of the mountains—the noble and hardy firs and pines—ripen their seeds. These trees abound in resin, and maintain their temperature above the freezing-point even in the severest weather. Their fluids are never congealed, owing
to their viscidity, and they can, therefore, resist the cold. The sap continues to ascend in them, although its upward flow is very considerably diminished.
There appears, also, to be some continuance of vital activity amongst the evergreens which bear true leaves-such as the ivy, laurel, and holly. From the very nature of things, these plants must change their leaves; but they do it in a less rapid and visible manner than the deciduous-leaved trees, one leaf replacing the other in such a way that the tree is never totally deprived of foliage. In the spring of the year there is a partial leaf-fall from the branches of evergreens. The leaves of these plants are more or less in action during winter.
A low degree of warmth will, even in the depth of winter, start the sap of plants. Thus, if incisions be made into the stem and branches of a young maple in winter, if the weather should become mild, the sap will be seen to trickle from the wound.
As a general rule, however, winter is a state of repose to all the higher Phanerogams, or flowering-plants, the only really active portions of vegetation being Coniferæ, Evergreens, and the lower orders of the Cryptogamia. Yet, even if these higher flowering-plants manifest no active life-processes during their winter's slumber, one process is as absolutely necessary to their preservation as to their growth-there must be an interchange of matter between the air and the body of the plant. This exchange may be small
, but it is more than probable that it to some extent exists; for it is a law of plant-life that it can only exist so long as evaporation and breathing are carried on. In winter the inside of a tree, examined with a thermometer, shows a higher temperature than that of the surrounding atmosphere, and this warmth is a proof of vital changes. Although in the development of most deciduousleaved trees and perennial herbaceous plants, winter is a state of rest, yet they retain their vital properties, and it is no more a state of absolute inactivity than the healthy sleep of the animal body, with which this winter's repose of vegetation may be very properly compared.
Even those trees which are apparently devoid of foliage, still possess leaves. We refer to the scales or covering leaves which constitute the outer envelopes of the buds. These are the winter leaves of trees, and each tree has them quite as characteristic of its species as its summer leaves. That the scales of buds are only modified leaves, every botanist will readily allow. In many plants the transition of the bud-scales into ordinary green leaves is well marked; and that they are varieties only of the same common leaf-forms, is also indicated by their spiral arrangement, and by the fact that they are alternate or opposite in position about their rudimentary axis, exactly as the leaves may happen to be arranged on the fully developed shoot. These bud-scales or covering-leaves must be regarded as the very lowest type of leaf. They begin to form in spring, and continue growing through the whole of the summer. During the active period of vegetation the current of sap is diverted away from them by the summer leaves, so that they cannot make much progress. In autumn the sap gradually ceases its flow towards the summer leaves, and its last movement, before it stagnates in the tissues, is probably to these winter leaves or bud-scales ; for it is then that the buds are matured, just before the trees shed their summer leaves in autumn. Hence physiologists have very properly designated as autumnal sap this peculiar flow of the stagnating nutritive
The winter leaves being formed under such circumstances, are necessarily circumscribed to the smallest space, and as they do not take the conspicuous forms of the summer Jeaves, they escape vulgar observation.
These winter leaves of trees, so apparently insignificant, will, however, richly repay investigation. The tree requires protection, not nutrition, in winter, and these leaves are organized for that purpose. They are devoid of chlorophyll or leaf-green, and without pores. In some instances their surface is clothed more or less densely with hairs, or it is covered with glands which exude a copious resinous or glutinous secretion, with which the surface of the bud is overspread. These leaves are thus organized in order that they may protect the next year's foliage, blossom, and fruits. All the beauty and glory of the vegetation of the coming year is left in the charge of these inconspicuous bud-scales or winter's leaves; for every leaf and blossom is already formed and securely packed away into the smallest possible space in these buds, under the air and water-tight roofs formed by them. In order to verify these facts, it is only necessary to dissect one of these buds, by removing its scales, and examining the embryo leaves in its interior with a microscope. The bud of the horse-chestnut is one of the best that can be selected for this purpose. The embryo leaves will be found in the interior of this bud, in a warm bed of tomentum or down, packed away securely for the winter.
The winter leaves or bud-scales continue attached to the trees throughout the winter months; but when spring comes, and the summer leaves which they have protected are pushed out into the air, the tree shakes off its winter leaves on the green grass carpet, exactly in the same way as it does, later in autumn, its summer leaves. So from all the trees of the forest there is every year a double leaf-fall. The fall of these winter leaves is beautifully seen in the beech, maple, and horsechestnut; and, as we have already remarked, an observer of nature will experience no little gratification in studying these little obscure though important folioles. The bud was called by Linnæus the hybernaculum or winter-quarters of the shoot; and buds with the winter leaves on their exterior may also be regarded as their winter defences. The winter leaves are organized to protect the life slumbering in the buds against the cold of this season of the year. A low temperature appears to be necessary to their life, for as soon as the warm days of spring commence and the summer leaves come out, they fall. from the tree. These winter leaves cannot, therefore, live under the same circumstances as the summer leaves; and the winter atmosphere in which they live and discharge so faithfully their protective duties to the summer leaves would destroy the life of the latter.
As the summer leaves make on the stem a cicatrix or scar which shows their former point of union, so with the wintery leaves; these also leave their mark or scar; only in this instance, the internodes or naked intervals of stem between the leaves are suppressed, the scars are necessarily close together, and