18S9] 



FARMERS' REGISTER, 



171 



dullarv rays, and are considered a part of the pitli. 

 Like the pith they are composed ol cellulnr tissue ; 

 their more solid appearance arisinjr Iron) the strong 

 lateral pressure to which the cellules have been 

 suhjecied, hy being interposed between pistes of 

 wood; thus givmg to those cellules the form of 

 thin parallelograms. These rays are channels 

 of coiniiiunicalion between the bark and the inter- 

 nal portioiis of the stem. Through them, certain 

 of the claliorated juices which descend m the bark 

 arc carried iu and deposited in ihe wood. 



The wood, which exists more or less abundant- 

 ly, even in herbaceous stems, and which forms so 

 large a portion of those of trees and shrubs, in 

 the stem which we have selected for examination, 

 consists of a single zone or layer, composed of 

 tubes and woody fibre, disposed without any re- 

 gular order, except that the latter is the most 

 abundant on the outside, next the bark. The se- 

 cond yearof a plant's growth, a new layer is form- 

 ed outside of the first, and similar to it in every 

 respect. The third year this process is repeated ; 

 and thus the stem increases in size, a new layer 

 being lormed annually, as long as the plant lives. 

 The wood of an exogen of one year's growth, may 

 be viewed as an elongated hollow cone, extending 

 from the base to the summit of the stem, and en- 

 closing the pith. This cone does not extend fijr- 

 (her, nor does it enlarge in anj' way ; but is sur- 

 rounded, the next year, by another cone, which, 

 like the first, alter being formed, undergoes no 

 change in dimensions. Hence as the necessary 

 result of this mode of growth, the stem of an ex- 

 ogen is more or less conical. 



As each layer, or to speak more accurately, 

 each hollow cone of wood, is the result of a single 

 years growth ; it is evident that the age of an ex- 

 ogen may be ascertained by counting the number, 

 or rings presented on a transverse section of the 

 stem, made near its base. This may be done with 

 great accuracy, in most trees of temperate and cold 

 climates, in which, in consequence of the periodi- 

 cal suspension of vegetation, the annual layers are 

 distinctly marked ; but in the case of trees of the 

 torrid zone, where vegetation goes on ihoughont 

 the year, this cannot be so readily done. In old 

 trees, the rate of increase being very uniform, 

 their age may be determined with considerable 

 accuracy, by the inspection of a mere fragment of 

 the stem, the diameter of the whole stem from 

 which it was taken bemg known. A rough esti- 

 mate of the age of a tree, is sometimes made, by 

 dividing the semi-diameter of its base by the ave- 

 rage increase of the species to which it belongs, 

 that average being determined by previous obser- 

 vation. In these several ways, the ages of nume- 

 rous very old trees have been determined. It 

 should be remarked, however, that the determi- 

 nations, except where they are based upon an ac- 

 tual countmg of the rings presented by a trans- 

 verse section of the trunk, cannot be regarded as 

 any thing more than approximations to true age. 

 A tree growing in peculiarly fertile ground, will 

 enlarge much more rapidly than most other trees 

 of the same species; and of course, with a given 

 diameter, will have a less number of zones than 

 the average. In the case of a tree growing in 

 peculiarly barren ground, just the opposite effect 

 would ensue. An estimate of the age of the first, 

 made by dividing its eenii-diameter by the ave- 

 rage ihicknese of the zones of that particular spe- 



cies, would give too great an age. An estimate 

 of the age of the last, made by the same method, 

 would give an age less than the true one. 



Another, and far more important source of error 

 in the case of m.any very large trees, is to be found 

 in the fact, that several stems, starting from the 

 same root, sometimes become so thoroughly uni- 

 ted, as to present to the eye the appearance of a 

 single stem. Such, according to the testimony of 

 most travellers, is the fact with respect to the cel- 

 ebrated sweet chestnut of Mount Etna ; measur- 

 ing 180 feet in circumference, or about 60 feet in 

 diameter. Whilst speaking of the union of dif- 

 lerent stems, 1 cannot forbear mentioning a curi- 

 ous insiance of it, which I had pointed out to me 

 some time since. It was in the case of two trees 

 srowiniT two or three leet from each other. At a 

 height of 14 or 15 feet from the ground, one of 

 them had sent out a branch, which had become 

 thoroughly incorporated with the trunk of the 

 other. After this had taken place, the second tree 

 had gradually decayed, between the point where 

 this limb was inserted and the ground, until near 

 the root, it was entirely dead. Yet at the time I 

 saw it, several of its upper limbs continued to 

 thrive, deriving all their nourishment from the 

 roots of the second tree, through this branch. 

 This may perhaps be considered an instance of 

 natural grafting. 



There is almost always a marked difl^erence in 

 color and density, between old and recent wood. 

 The outer and more recent portions of the stem, 

 iiave been called, in allusion to their color, albur- 

 num ; and in allusion to their office, sap-wood ; the 

 inner and older portions are termed the heart- 

 wood. After a fi^w years, the color of a layer of 

 wood is changed, its density is increased, and it 

 takes thereafter little part in the transmission of 

 the sap. During the winter, it is true, it generally 

 contains sap, but then this sap is rather deposited 

 in it, than circulating through it. The change in 

 color and density, by which sap-wood is converted 

 into heart-wood, is caused by the deposition of a 

 solid matter, peculiar to each species, in the tissues 

 of that part. This matter is in most cases soluble 

 in nitric acid, and hence ii is, that if a piece of 

 heart-wood be immersed in that acid, the color is 

 discharged, and the piece again assumes the ap- 

 pearance of sap-wood. Where the matter de- 

 posited is a resinous character, as in the pines, it 

 adds very nmch to the durability, and consequent- 

 ly to the value of the heart-wood. On this ac- 

 count, as well as on account of its greater solidity 

 and strength, the heart-wood is universally pre- 

 f(3rred to the sap-wood, lor use in the arts. As 

 the layers of wood, in the course of a few years 

 aller their formation, cea>e to take any active part 

 in the circulation of the sap, and, in time, become 

 to all intents and purposes dead matter, it would 

 naturally follow that the central part of the stem 

 would be first to decay. Where the matter de- 

 posited in those parts is not of such a character as 

 to protect them from decay, this is frequently found 

 to be the case. The resinous matter, deposited in 

 the heart-wood of the pine, is of such a nature as ef- 

 lectually to resist the disorganizing agencies which 

 operate upon it; and hence it is that pines, even 

 those of the greatest age, are never found hollow. 

 But such is not the fijct with respect to the matter 

 deposited in the heart-wood of" the sycamore, 

 Cplantanus occidentalis,J and hence all the oldest 



