Изображения страниц
PDF
EPUB

upper and under limit of the rings, shows the amount of shoot developed during the season. The variability of these distances, or year's growths, is seen by one glance at the plate. The growth of the primary axis appears to have been accelerated and retarded five times during the thirteen years, or between the years 1851, 1854, 1857, 1859, and 1862.

A similar amount of variability is visible in the yearly growths made by the first and second side-shoots. In these two shoots, and in the central axis or stem of the twig, there is evidently the greatest amount of active vitality, and consequently their fluctuations of growth are the most visible and variable, beyond those of any other shoots in the series.

And here we would call the attention of the reader to the somewhat remarkable circumstance, that the yearly fluctuation of growth of the second or largest shoot corresponds with those of the central axis of the twig, upon which it would seem to have been dependent, whereas the first shoot or the one situated immediately below it has a fluctuation in its growth which is entirely different from that of the central axis, by which its growth does not seem to have been at all affected.

In the other shoots of this twig, vegetative life is approaching to a state of torpor or inactivity, and hence there is a much greater amount of uniformity, not only in their yearly growths but also in the number of leaves annually produced. In the fourth shoot, numbered 10, for example, the growth and foliage appears to be approaching a minimum. For ten successive years this shoot has only unfolded its terminal bud, thrown off its winter leaves, put forth two leaves the vital activity of which has been so enfeebled that they were only able to form two lines of clear shoot annually, between the successive sets of annuli, with which its surface is almost entirely covered. This shoot is also curved, which is another indication of its fast-failing powers of life, which the other shoots do not manifest. When shoots curve in this manner, it is nearly all over with them. A year or two more and the terminal bud loses the power of throwing off its winter leaves, because the summer leaves no longer form in its interior; the bud then dies. With the death of its terminal bud and the cessation of the formation of any more leaves, the further growth of this shoot must necessarily be completely arrested.

But not only the winter leaves, but also the summer foliage leaves behind it a peculiar scar on the bark, which marks the point of the stem to which the leaves were attached, and the number of leaf-scars between two sets of annuli therefore enables us to estimate correctly the number of leaves which were put forth during that season.

Now there are fifty-four leaf-scars on the central axis of our

twig, many of which are represented in our engraving and may actually be counted in the plate if a small lens is used, such is the accuracy with which nature has been copied. The lines across the central axis indicate the position of these leafscars, for they are left by the stipule which was connected with each leaf as an enveloping and protecting organ. The fifteen branches, of course, originated from fifteen leaves. For instance, there are two lines visible, and three branches, between 1850 and 1851, on the central axis, which proves the attachment of five leaves to that portion of the axis. The first and second branches and the whole of the twigs with its shoots, above fifty-one, were then buds. Those buds indicate the position of three leaves which flourished during that season and brought them to maturity in autumn.

Again, there are altogether 299 leaf-scars on the sidegrowths of our twig, which proves it to be the work of 299 leaves. Evidence of the former presence of many of these leaves may be seen also with a glass on these side-growths in the lines and rudimentary buds left between the sets of annuli on their surface. Therefore the total number of leaves by which the entire twig was constructed was 54+299=353.

The following is a true estimate as to the size of the twig. The length of the primary axis is twenty inches and three lines, and of the largest secondary axis ten inches and eight lines, yet it is the leaf-labour of 353 leaves. The twig itself we have shown to be only thirteen years old. What then must be the immense number of leaves engaged in the construction of trees which put forth thousands of such twigs, which grow from one to two hundred feet in height, and whose giant forms have stood for hundreds and even thousands of years? Who can estimate the quantity of leaf-surface spread abroad in the atmosphere from the first commencement of growth, and the amount of leaflabour necessary to rear such lofty and enduring vegetable monuments? The Washington elm at Cambridge-a tree of no extraordinary size-was some years ago estimated to produce a crop of seven millions of leaves, exposing a surface of two hundred thousand square feet or five acres of foliage.*

The number of leaves which a tree puts forth during the season is undoubtedly regulated by the size of the dilated portion of the leaf, that is to say, its lamina or blade; if that be narrow the leaves are found in greater numbers on the tree. Compare in this respect the quantity of leaves on a willow with those on a horse-chestnut or maple. But this law is most apparent when the lamina or blade of the leaf is abortive, as in the linear needle-like leaves of the fir and pine.

* "First Lessons in Botany & Vegetable Physiology." By Asa Grey. 1857.

These make up by their immense numbers and their persistent nature for their want of surface, and thus contribute to the growth of the branches of these trees, as completely as those which possess a true lamina or blade.

Again, the increase of leaf surface each year, as ascertained by calculations made on the original twig, was as follows:1851, 5; 1852, 10; 1853, 14; 1854, 15; 1855, 23; 1856, 28; 1857, 29; 1858, 36; 1859, 38; 1860, 37; 1861, 36; 1862, 40; 1863, 44. That is to say, in 1851 the twig put forth five leaves; in 1852, ten leaves, &c., &c.

It is worthy of remark here, that with every increase in the number of its absorbent surfaces or leaves, a young tree or branch must necessarily grow more rapidly. At any rate this law holds good up to the time when the tree begins to arrive at a maximum in its height and in the spread of its boughs; for till then the amount of leaf surface put forth in the atmosphere increases continually. When this period approaches, however, the vitality of the leaves put forth on the shoots at the extremities becomes enfeebled, and the shoots themselves become every year more and more circumscribed in their growth. The annual crop of leaves is now pretty much the same year after year. But the lofty tree, though it stands for hundreds and even thousands of years, has an appointed limit to its life, like the lowly flower that grows beneath its shade. Sooner or later it will begin to die at its extremities and decay in its interior, until at length the stem itself is merely a hollow living shell of wood, constituting the only bond of connection remaining between the roots and the branches which are still covered with verdure. As, however, branch after branch dies, the annual amount of its leafage necessarily diminishes. It is, however, extremely difficult to point out the several stages of stagnating and expiring growth, or to prove that a tree will ever die. In most cases death is brought about by violent interruptions to the natural life-processes. After having braved the storms of a thousand years, the tree is at last blown down and uprooted. Now busy, active, ever-industrious nature covers its fallen mouldering trunk with a shroud of moss and lichen, and there it lies in that forest grave-yard until it is again resolved in its original elements of earth and air.

The twig represented in the plate shows beautifully this gradual arrest of growth at its extremities, and also the diminution of annual produce in the foliage; thus illustrating clearly that law which governs the whole tree as it approaches the close of its allotted period of life. Let us confine our observations to the primary axis of our twig, which, with its little sideshoots, represents the massive stem and branches of the tree from which it was cut, on a smaller scale of architecture, being

precisely the same in anatomical construction and governed by the same laws.

The tree from which this twig was cut was one of the oldest in the forest, and must have been growing for several centuries; the twig itself had only been growing for thirteen years, yet a transverse section of the primary axis of the twig, if compared with a cross-section of the stem of the tree, would show, notwithstanding their difference in size, the same parts, the pith in the centre, a number of concentrical rings of wood, and the bark on the outside. But in the twig, there would be only thirteen rings of wood, in the stem of the tree several hundreds. For each tree, each twig carries in its interior the monumental history of its life, and exhibits in the cylindrical woody deposits of each vegetative season marks which are ineffaceable excepting from the influences of decay.

Between the years 1850 and 1859, or for the first nine years, the growth of the central axis was nineteen inches and four lines; also, in 1851, the primary axis, which was then without a single shoot, only produced itself five leaves; but, in 1859, it had put forth eleven shoots, all of which were in foliage, making the entire leaf crop thirty-eight, or an addition of thirtythree leaves to the annual produce in eight years. But the growth of the primary axis, between 1859 and 1863, was only two inches and seven lines, with an addition to the annnal leaf-crop of only six leaves, and this, too, in four years. Our twig was, therefore, cut when it was approaching the limit of its vegetative progress, for its growth annually was stagnating, as is evident from the approximation of the sets of annuli at its extremities, and the slower increase in the annual extension of its leaf surface.

An animal may continue to live after it ceases to grow, but it is not so with a tree or a twig. For a tree or a twig continues to grow as long as it lives, and when it ceases to grow it ceases to live. This gradual and continuous stagnation of growth at the extremities of our twig is, therefore, also indicative of the fact, that its vegetative force was gradually expiring, and also shows that its life was rapidly drawing to its close.

Once more, we have seen that fifty-four leaves were employed in constructing the primary axis, yet only fifteen of them produced buds which became branches; also, that these fifteen branches, although constructed by 299 leaves, developed only fourteen shoots; for, referring to the plate, the reader will find that the first side branch produced three; the second, nine; and the seventh, two shoots; it follows that of these 299 leaves only fourteen produced vitally active buds, and that the axilla of all the others remained unfruitful. Therefore, the total number of abortive or rudimentary buds in the entire twig must be 353 −(15+14) = 824.

Some of these 324 buds have retained a sufficient amount of vitality to carry them forward through the annually deposited layers of wood and bark, so that they still continue to maintain their position, year after year, on the outside of the bark. Several of these buds are visible in our plate on the surface of the central axis of our twig, and also on that of its first and second side shoots, and more may be detected on the surface of the other shoots, if the plate be examined with a small magnifier.

For example, there is a rudimentary bud on the primary axis, just below the set of annuli, marked 52, on the left side of the twig, which was matured by a leaf which fluttered in the winds and was bathed in the showers of the spring and summer of 1851; that leaf fell from off the axis in autumn, and left that bud on its surface, which has just life enough left in it to enable it to keep on the outside; that bud has been in that torpid condition for twelve years. And the bud, just above it, on the same side, nearly opposite that curved shoot, which is so instructive, was matured in the autumn of 1852, and has, therefore, been torpid for eleven years.

In cases like these where a bud has been formed by a leaf which has died years ago, and has maintained its position on the outside, if a section be made at the point of the stem where it is seen to protrude, the vegetative course of the bud will be marked by a line of pith, which traverses the several layers from the centre outwards.

In other instances the abortive buds of our twig died and were detached from the shoot the first year, or they retained their life but continued totally inactive. In the latter case the abortive bud necessarily sank below the surface, and became buried beneath succeeding annual deposits of bark and wood.

The trunks and branches of trees always contain an immense number of these buried buds, which may remain for years entombed below their bark in a state of passive vitality, like seed which is buried in the ground.

Leaving out of consideration the rudimentary buds visible to the eye unaided, or assisted by a magnifier, on the surface of our twig, and supposing one half of the rest to have died, it is probable, even then, that there are more than a hundred abortive buds or points of vegetation buried beneath the bark of this little twig, only 13 years old, and 20 inches in length! What, then, must be the immense number of abortive buds in powerful branches which have been growing for centuries, or in the stem of a tree built by the leaf-labour of a thousand years. The vitality of those buds is not destroyed. Their parent leaves, it may be, have died and dropped from the tree many, many years ago, but they still retain unimpaired the life which they then received. Let some of the leading

« ПредыдущаяПродолжить »