Cedars and junipers make an especially effective wind-screen, and on the eve of a bitter night little birds gather in numbers on the branches of these trees, close to the trunk.
The habit of growth of the cone-bearers is similar to that of the oaks and maples and other kindred of the rose. The ascending stream of water from the roots passes through the younger wood, while the descending stream of sap from the leaves moves through the inner bark. The tree grows thicker as it grows older, and between bark and wood, each growing season, there is a ring of actively-dividing cells which are building up new tissue. The oldest wood is at the centre of the trunk, and the newest is just beneath the bark (Fig. 78).
But in many cone-bearers and notably in the white pine, the heart-wood undergoes little alteration as the tree matures, and it can resume the industry of former years, if necessary, and conduct water upward toward the thirsty leaves. Indeed so great is its versatility that it can make shift to fill, after a fashion, the offices of young wood and of bark, so that plant-fluids still ascend and descend slowly even in a girdled pine.
Fig. 78. - Crosswise section of the trunk of a fir-tree, showing growth-rings.
(From the Vegetable World).
In addition to this capacity of their wood for meeting an emergency, the cone-bearers have another peculiarity which helps them to survive misfortune.
For in all our native evergreens except the yew, the wood contains a quantity of resin. In the living tree this resin is held in solution in oil of turpentine, and the two together make a clear, sticky fluid known as "balsam." In the larch, pine, and fir there are little wells of it in the trunk and branches, and sometimes even in the leaves. The balsam of the fir is so abundant and adhesive that the Canadians and Indians made use of it for tightening the seams of their canoes.
" Give me of your balm, O fir-tree," cries Hiawatha, "Of your balsam and your resin, So to close the seams together, That the water may not enter. And the fir-tree, tall and sombre, Sobbed through all its robes of darkness, Take my balm, O Hiawatha".
The balsam pours out wherever the wood is wounded, and, by exposure to air and sun, it stiffens and forms a plaster for the torn tissues. This preserves the life of the wood, which, if left unprotected, would soon have all its vital juices dried away.
So girdled cone-bearers have been known to exist for forty years. Indeed a pine has "as many lives as a cat." We realize this when we see the pitch-pines at home, in the "turpentine country" of Georgia. Deeply wounded, or even girdled, and all bare save for a tuft or two at the top, they still live, and remind one of Charles the Second who was "such an unconscionable time a-dying.'
Were it not for these peculiarities of structure, girdled pines would share the fate of girdled-oaks and maples, which seldom survive their injuries for more than three or four years. In these trees the heart-wood, which has retired from active service, can never resume its conductive duties, and there is no balsam which can be converted into sur-geons'-plaster in time of need.
So the wood which is laid bare dries out more and more, and as soon as the drying has penetrated the outer or vital part of the trunk plant-fluids can no longer move between leaves and roots, circulation stops, and the tree dies.
Though the cross-section of a pine-tree is much like that of an oak, their woody tissues have a different aspect under the microscope.
The wood of the cone-bearers is almost entirely composed of "tracheids," which are little tubes tapering to a point at either end (Fig. 79). Most of these run lengthwise of trunk or boughs, and in their walls there are circles or ovals at regular distances apart. Each of these is a little plate of very thin tissue, set into the partition between two tracheids, and framed on both sides by a ring-shaped bulge in the tracheid-wall. The whole affair looks like a tiny circle surrounded by a halo. When the tracheids were young and full of protoplasm, plant-fluids were drawn through the thin spots, and thus a vital communication was kept up through all the maturing tissue. But by the time the tracheid is fully developed the protoplasm which has filled it disappears, and the mature wood of a cone bearer contains little else but a film of water on the tracheid-walls. So most of the "bordered pits" are no longer useful in the vegetable economy.
The Coniferae combine the utmost grandure of form with the greatest simplicity of floral structure. They are among the earliest terrestrial plants known to us. They are "the seniors of the forest," - surviving types from a younger world. They were many and prosperous in the geologic "Age of Reptiles," when animal life swam and crawled but had scarcely yet begun to run or fly.
Fig. 79. - Tracheids of the fir-tree. (Magnified).
The first flowers the young world saw were borne by the Coniferae, and as there were no winged insects in those days, the trees had to send pollen to one another by the wind. Now, when the summer air is full of possible pollen-carriers ready for errands, and when less conservative flower-families have learned to rely altogether upon their ministrations, the Coniferae depend, as of old, upon the wind alone. They are like the people of some unprogressive communities, who cling to old methods of work, and look askance on modern machinery and labor-saving devices.
As the Coniferae can carry on their affairs without the aid of flying messengers, they are able to perpetuate themselves abundantly in cold regions, while gay blossoms, which cannot set their seed without the ministrations of insects, are practically restricted to latitudes where the climate is favorable to the life of their winged friends.