38

INSCRIPTIONS BURIED IN WOOD.

first injured the tree been so bent on its destruction as to cut away the newly-formed tissue, union would have speedily been effected, and the tree in all probability preserved. The growth of new tissue was not assisted by any thin strips of the inner bark still adhering to the tree, by which the descending tissue could have been conducted, but proceeded simply as indicated above, from the medullary rays."

M. Trécul has shown (Annales des Sciences Naturelles, Oct. 1853) that the denuded surface of the young bark (in the Elm for example) is no less capable of giving rise to a similar growth, and this whether the strips of bark separated from the stem are torn upwards or downwards, and are connected therefore with the tree above or below. New wood and bark may also be formed where there is not a single leaf, as in the case of vigorous trees cut off level with the ground. Many such instances are on record, but none more remarkable than that described by Goeppert in the Silver Fir (Abies picea, &c.) In some cases of this sort there was an inosculation with the roots of other trees; in others no such inosculation was possible.

The manner in which figures or letters carved in trees are gradually filled up affords another example of the process in question. Of this the following striking instance is illustrated in the Gardeners' Chronicle of 1841, by Professor Henslow :

An Ash-tree in Coxwold, near Thirsk, was ordered to be felled and split for firewood. Upon being riven asunder, the outer part of the tree was cleft in two, like a case, leaving the inner portion of the trunk entire; and the rude inscription represented in the accompanying cut was discovered, distinctly legible, both upon the inner part of the trunk, and with the letters inverted, upon the outer casing.

There is no date to the inscription, but the period at which it was made may be ascertained, with much probability, from the following considerations. The tree is deposited in the Museum of the Hospital at Kirk Leatham, between Stockton-upon-Tees and Redcar. The porter of the Hospital, now living, can vouch for its having been there upwards of seventy years; and the tradition respecting the tree is, that it was given by Lord Falconburg, from his manor at Coxwold, to Mr. Cholmley Turner, who died on the 9th of May, 1757. It would therefore appear that the tree had been cut down nearly a hundred years. Also, by the number of rings in the wood, each indicating a year's growth, the tree appears to have been about fifty-five years old when the inscription was made, and to have subsequently grown for nearly two hundred years. The closeness of the rings near the circumference renders it highly probable that the inscription was made about three centuries ago. The height of the fragment of the tree is 5 feet 4 inches. The circumference of the inner block measures, at the upper part, 2 feet 1.5 inches; and at the lower part, 2 feet 10.75 inches: that of the outer block measures 4 feet 8.5 inches at the upper part, and 6 feet

perpetuating and discovering the inscription. As the tree continued to grow, new wood would be formed between the inscription and the bark; and thus the record became buried for centuries in the heart of the tree.

In the bark of trees and shrubs, two distinct parts are found the one external and cellular; and the other internal, resting upon the wood, and consisting of woody matter mixed with cellular. The external is the RIND or cortical integument,

40

HEARTWOOD AND SAPWOOD.

the internal is the LIBER. These two parts grow independently of each other, by their inner faces; the rind belonging exclusively to the horizontal system, the liber composed of the perpendicular and horizontal systems intermixed.

In all Exogenous plants whose stems acquire an age beyond that of a very few years, the wood is distinguishable into two parts, heart-wood, and sap-wood or alburnum. The former is more or less central, and coloured brown or some other tint; the latter is external, pale yellow, and much softer. Heartwood was originally alburnum, and altered its nature with age, in consequence of the solid matter with which all its tubes and vessels were choked up; alburnum is the youngest wood, with all its communications free and open, no solid matter having had time to accumulate within them. The reason why solid matter collects in the tubes of wood, so as gradually to choke them up, is this: the wood is the channel through which all the fluid matter of a plant, whether crude or digested, passes, in its way upwards to the leaves, or in its horizontal direction from the bark to the central parts of the stem. When sap leaves the earth and passes into the stem, it ascends by the woody matter of the finest fibres of the root having left them, it flows into the new wood with which those fibres are connected, and passes along this until it reaches the leaves; on its return from them it descends through the liber, in part passing off horizontally towards the centre through the medullary rays. Wherever it passes it deposits a portion of its solid parts; and, consequently, that portion of the wood, namely, the oldest or the heart-wood, through which it has passed the most frequently, will have the greatest quantity of matter accumulated within it, independently of all other reasons for its hardening.

In consequence of their peculiar manner of growth, new living matter being continually formed near the circumference of the trunk, and over that which is older, Exogenous trees arrive at an old age wholly unknown in the rest of the creation. And although some of the statements on this subject may be exaggerations, yet as there is a certainty that some individual trees have lived for more than one thousand years, so it is quite possible that others may have existed for a very much

LONGEVITY OF PLANTS.

41

longer period. It is, however, not probable that Exogenous trees have a power of indefinite life. Upon this subject the following remarks by Professor Mohl are among the best which have been made :

"The peculiarity of their organisation, and the unlimited power of growth of plants, offer many difficulties to the definition of the duration of plants, and have given rise to many incorrect theories. Every individual cell, and every individual organ, has a determinate end to its life; but the entire plant has not, since the individual shoots run through their periods of development quite independently, and only share in the weakness of age of the older organs when these are no longer able to convey to the young shoots the needful amount of nourishment, in which case the latter do not die from deficiency of vital energy, but are starved. It therefore depends wholly upon the mode of growth of a plant whether this occurs or not. When a plant possesses a thallus spreading horizontally by the growth of its circumference, it can annually extend itself into a larger circle, after the old parts in the centre have been long decayed, as is seen in old specimens of crustaceous Lichens, in the fairy rings caused by Fungi, &c. In like manner when a higher plant has a creeping stem, and possesses the power of sending out lateral roots near the vegetating points, and in this way conveys nourishment directly to the young terminal shoots, the latter are wholly independent of the death of the older parts of the stem and of the primary roots, and there exists no internal cause for death in such a plant. It is truly a different plant every new year and vegetates in a new place, but there is no definite boundary between it and its predecessors; such a plant is like a wave rolling over the surface of a sheet of water; it is every moment another and yet always the same. Thousands of inconspicuous plants, of Mosses, Grasses, Rushes, &c., have vegetated in this manner upon peat bogs and similar localities perhaps for thousands of years. Plants with upright stems are placed in much more unfavourable circumstances. It has been declared of these also, and particularly of the Dicotyledonous trees (De Candolle, Physiologie Végétale, ii. 984), that they have no internal cause for death, but I believe incorrectly. Examples of very old trees, such as De Candolle collected (e. g., Taxus 3000, Adansonia 5000, Taxodium 6000 years old, &c.), only prove, naturally, that death occurs at a very late period in many plants placed in favourable circumstances, but not that it does not necessarily happen. To me there appears to exist in all trees, whether they belong to the Dicotyledons (Exogens), or, like the Palms, to the Monocotyledons (Endogens), an internal cause which must produce death in time-namely, the increasing difficulty of conveying the necessary quantity of nourishment to the vegetating point, resulting from the elongation of the trunk from year to year. Even when the force which carries the sap up, suffices to raise it to two hundred feet or more (many Palms, as Ceroxylon andicola, Areca

42

LONGEVITY OF PLANTS.

oleracea, attain a height of one hundred and fifty to one hundred and eighty feet; some Coniferæ, e. g., Pinus Lamberti, Abies Douglasi, of more than two hundred feet), yet a maximum is reached there, and the terminal shoot is less perfectly nourished every succeeding year, becomes stunted more and more, and the tree at length dies. If we are surprised at the intensity of the vegetative force of individual plants, in consequence of which it re-appears with new, unweakened energy in every bud, so must we marvel at the force committed to so simple an organ as a cell is, if we reflect what an influence it exerts upon the total economy of nature, as one of the grandest of phenomena. The plant lives almost solely upon inorganic substances; its cells are chemical laboratories in which these are combined into organic compounds. The plant prepares in this way not only the nutriment required for its own development, but also the food on which the entire animal kingdom depends. But plants not only nourish animals, they maintain the air in a fit state for their respiration, since their breathing process removes carbonic acid from the atmosphere and replaces it by oxygen gas. In all these functions the plant is thoroughly dependent upon the outer world; its food is brought to it without its own cooperation by water and air; its respiration takes place without activity of its own, through a penetration of its substance by gases with which it is in contact, in consequence of a physical law; not even does its internal circulation of juices depend on a mechanical activity of a circulating system; thus every necessity for motion is removed. It is true we here and there meet with movements in this or that organ, but these, occurring isolated in the vegetable kingdom, are also altogether of subordinate kind in the individual plant."

The stem of a plant consists, then, of the following parts, viz. : 1. Wood, the oldest of which is heart-wood, and the newest alburnum; this is the substance through which sap ascends: 2. Bark, the external coating, down the liber or inner face of which sap descends: 3. Pith, a central portion of the horizontal system: and, 4. Medullary Rays, serving to connect the bark with the pith, to hold all the parts together, and to maintain a communication between the centre and the circumference of a stem. The stems of all plants have these four parts more or less evident. They are most visible in European trees or shrubs, in any of which they can be distinctly observed; they are least apparent in annual and herbaceous plants, because their lines of separation are not defined, all the four parts adhering to each other so firmly as to render it difficult to