« PreviousContinue »
was originally colourless, and owes its different tints to matter deposited gradually in all parts of the tissue; as may be easily proved by throwing a piece of heart-wood into nitric acid, or some other solvent, when the colouring matter is discharged, and the tissue recovers its original colourless character. That part of the wood in which no colouring matter is yet deposited, and consequently that which, being last formed, is interposed between the bark and duramen, is called alburnum. The distinction between these is physiologically important, as will hereafter be explained.
Each zone of the vascular system of an Exogenous stem being the result of a single year's growth, it should follow that, to count the zones apparent in a transverse section is sufficient to determine the age of the individual under examination; and further, that, as there is not much difference in the average depth of the zones in very old trees, a certain rate of growth being ascertained to be peculiar to particular species, the examination of a mere fragment of a tree, the diameter of which is known, should suffice to enable the botanist to judge with considerable accuracy of the age of the individual to which it belonged. It is true, indeed, that the zones become less and less deep as a tree advances in age; that in cold seasons, or after transplantation, or in consequence of any causes that may have impeded its growth, the formation of wood is so imperfect as scarcely to form a perceptible zone: yet De Candolle has endeavoured to show in an able paper, Sur la Longévité des Arbres, that the general accuracy of calculations is not much affected by such accidents; occasional interruptions to growth being scarcely appreciable in the average of many years. This is possibly true in European trees, and in those of other cold or temperate regions in which the seasons are distinctly marked; in such the zones are not only separated with tolerable precision, but do not vary much in annual dimensions. But in many hot countries the difference between the growing season and that of rest, if any occur, is so small, that the zones are as it were confounded, and the observer finds himself incapable of distinguishing with exactness the formation of one year from that of another. In the wood of Guaiacum, Phlomis fruticosa, Metrosideros polymorpha,
and many other Myrtaceæ, for instance, the zones are extremely indistinct; in some Bauhinias they are formed with great irregularity; and in Stauntonia latifolia, some kinds of Ficus, certain species of Aristolochia, as A. labiosa, and many other plants, they are so confounded, that there is not the slightest trace of annual separation. It is also to be remarked, that in Zamias we seldom find more than two or three zones of wood, whatever may be the age of the individual; and yet it appears from Ecklon's observations, that a Zamia, with a trunk only four or five feet high, can scarcely be less than two or three hundred years old. (Lehm. Pugill. vi.)*
With regard to judging of the age of a tree by the inspection of a fragment, the diameter of the stem being known, a little reflection will show that this is to be done with great caution, and that it is liable to excessive error. If, indeed, the zones upon both sides of a tree were always of the same, or nearly the same, thickness, much error would, perhaps, not attend such an investigation; but it happens that, from various causes, there is often a great difference between the growth of the two sides, and consequently, that a fragment taken from either side must necessarily lead to the falsest inferences. For example, I have now before me four specimens of wood, taken almost at hazard from among a fine collection, for which I am indebted to the munificence of the East India Company. The measurements of either side, and their age, as indicated by the number of zones they comprehend, are as follows: —
Now, in the first of these cases, suppose that a portion of the side A. were examined, the observer would find that each
* According to Decaisne (Comptes rendus, v. 393.) the zones of wood in Menispermaceæ each result from the growth of several years.
zone is 0-225 of a line deep; and, as the whole diameter of the stem is 45 lines, he would estimate the side he examined to be 22.5 lines deep; consequently, he would arrive, by calculation, at the conclusion, that, as his plant was one year growing 0.225 of a line, it would be a hundred years in growing 22.5 lines, while, in fact, it has been only forty years. And so of the rest.
When we hear of the Baobab trees of Senegal being 5150 years old, as computed by Adanson, and the Taxodium distichum still more aged, according to the ingenious calculations of Alphonse De Candolle, it is impossible to avoid suspecting that some such error as that just explained has vitiated their conclusions.
To the characters above assigned to the stem of Exogenous plants there are several remarkable exceptions, some of which have been described by botanists; others are mentioned now for the first time.
Mirbel has noticed the unusual structure of Calycanthus (Annales des Sciences, vol. xiv.), in the bark of which, at equal distances, are found four minute extremely eccentrical woody axes, the principal diameter of which is inwards; that is to say, next the wood. The existence of this structure, noticed by the discoverer only in C. floridus, I have since ascertained in all the other species, and also in Chimonanthus. Gaudichaud attempts to explain this curious mode of growth upon the supposition that each leaf forms three fascicles of woody matter, whereof the central is the most powerful, and produces the mass of the stem; and the lateral ones, which are much weaker, give origin to the accessory axes; and he states, that in climbing Sapindaceous plants the same phenomenon occurs, only to a far greater extent. He represents that in those cases the fibres of each leafstalk separate into three or four principal branches, each of which applies itself to one of the internal woody axes of the stem, which, in time, consists of from four to eight distinct axes, the central being larger than the others, and each having its own cortical integument. The fact is curious, but I doubt whether the explanation is just. (Arch. de Bot., ii. 492.)
In Coniferous wood (fig. 35.) there is scarcely any mixture of bothrenchyma among woody tissue, as in other exogenous plants; in consequence of which a cross section exhibits none of those open mouths which give what is vulgarly called porosity to wood. Instead of this, the wood genenerally consists exclusively of that kind of tissue which has been described at p. 25., under the name of glandular, with the exception of the medullary sheath, in which spiral vessels are present in small num
bers. The Yew and Abies Douglasii are the principal exceptions in the former the woody tissue is the same as that of other Coniferæ; but many tubes have a great quantity of little fibres lying obliquely across them at nearly equal distances, sometimes arranged with considerable regularity, — sometimes disturbed as it were, so that the transverse fibres, although they retain their obliquity, are not parallel, — and sometimes, but more rarely, so regular as to give to the tubes of woody tissue the appearance of spiral vessels, the coils of which are separated by considerable intervals. The latter only is represented by Kieser, at his tab. xxi. fig. 103, 104.; but the former is by far the most common appearance.
In Cycadaceae the vascular system is destitute of vessels, as in Coniferæ ; their place being supplied by such bothrenchyma as has been already described at p. 22. But the zones of wood are separated by a layer of cellular substance resembling that of the pith, and often as thick as the zones themselves, while the pith itself is filled with bundles of fibro-vascular tissue. This structure is represented by Adolphe Brongniart, in the 16th volume of the Annales des Sciences.
Mr. Griffith has beautifully illustrated the structure of a plant called Phytocrene (fig. 36.), in Wallich's Planta Asiaticæ, vol. iii. t. 216. In this curious production the wood consists of plates containing vessels and woody tissue, having no connection with each other, and separated at very considerable intervals by a large mass of prosenchymatous cellular tissue
filled with vasiform tissue, and representing medullary rays. When the stem is dry, the woody plates separate from the other tissue, in which they finally lie loose.
In Nepenthes distillatoria the pith contains a great quantity of spiral vessels; the place of the medullary sheath is occupied by a deep and dense layer of woody tissue, in which no vessels, or scarcely any, are discoverable; there are no medullary rays; the wood has no concentric zones; between the bark and the wood is interposed a thick layer of cellular tissue, in which an immense quantity of very large spiral vessels is formed; on the outside of this layer is a thinner coating of woody tissue, containing some very minute spiral vessels; and, finally, the whole is enclosed in a cellular integument, also containing spiral vessels of small size. In this singular plant the outer layers are, it is to be presumed, liber and epidermis; and the cellular deposit between the former and the wood is analogous to cambium in an organised state, belonging equally to the wood and the bark. What is so exceedingly remarkable is the complete intermixture of the vascular and cellular systems, so that limits no longer exist between the two.
I have a specimen of the twisted compressed stem of a Bauhinia from Colombia (fig. 37.), in which there are no concentric circles, properly so called; but in which there are certain irregular flexuous zones, consisting of a layer of cellular
* It will be seen that the view I now take of the analogies of the parts in the trunk of Phytocrene is very different from that in the first edition of this work.