Dec. 22, 1887] 



NATURE 



183 



the annual rings, as shown in Fig. 2, which represents on 

 a larger scale a segment such as could be cut from a log 

 in the way described. It is clear from comparison of what 

 has been said, and of the two figures, that the " annual 

 rings " are simply the expression in cross-section of 

 cylindrical sheets laid concentrically one over the other, 

 the outermost one being that last formed. But on 

 examining the medullary rays in such a piece of timber 

 as that in Fig. 2, it will be noticed that they also are the 

 expression of narrow radial vertical plates which run 

 through the concentric sheets : the medullary rays are in 

 fact arranged somewhat like the spokes of a paddle-wheel 

 of an old steamer, only they differ in length, breadth, and 

 depth, as seen by comparing the three faces of the figure. 

 It is to be noticed that the medullary rays consist of 

 a different kind of tissue from that which they traverse, a 

 fact which can only be indicated in the figure by the depth 

 of shading. It is also to be observed that the "annual 



rings" show differences in respect to their tissue, as 

 marked by the darker shading near the boundary lines on 

 the outer margin of each ring. In order to understand 

 these points better, it is necessary to look at a piece of 

 our block of timber somewhat more closely, and with the 

 aid of some magnifying power. For the sake of simplicity 

 it will be convenient to select first a piece of one of the 

 timbers known as " deal " (firs, pines, &c.), and to observe 

 it in the same direction as we commenced with, i.e. to 

 examine a so-called transverse section. 



The microscope will show us a figure like that in the 

 woodcut (Fig. 3). There are to be seen certain angular 

 openings, which are the sections of the long elements 

 technically called trackeides, shown in elevation in Fig. 4- 

 It will be noticed that whereas along some parts of the 

 section these openings are large, and as broad in one 

 direction as in the other, in other parts of the section the 

 openings are much smaller, and considerably elongated in 



Fig. 2. 



Fig. 3. 



Fig. 2. — Portion of segment of wood from a log such as Fig. i, supposed to hz slightly magnified, a, annual ring ; m, medullary rays ; ;«', the same in 

 vertical sectiop ; c, the boundary line between one annual ring and another ; su, autumn wood ; sp, spring wood ; /, the pith. 



Fig. 3. — Portions of four annual rings from a thin transverse section of the wood of a Conifer, such as the Spruce-fir. M, a meduUarjr ray ; b and c show the 

 entire breadth of two annual rings ; a, autumn wood of an annual ring internal to 6 (and therefore older than b) ; d, spring wood of an annual 

 ring external to c (and therefore younger than c). Bordered pits are seen in section on somj of the tracheides. Magnified about loo limes. 



one direction as compared with the other. The band of 

 small openings naturally looks more crowded and there- 

 fore darker than the band of larger openings, and it is to 

 this that the differences in the shading of the annual 

 rings in Fig. 2 are due. But it is not simply in having 

 larger lumina or openings that the dark band of 

 tracheides is distinguished from the lighter one : the walls 

 of the tracheides are often also relatively thicker, and 

 obviously a cubic millimetre of such wood will be denser 

 and contain more solid substance than a cubic millimetre 

 of wood consisting only of the larger, thin-walled 

 tracheides. It is equally obvious that a large block of 

 wood in which the proportion of these thick-walled 

 tracheides with small lumina is greater (with reference 

 to the bands of thin-walled tracheides) will be closer- 

 grained, and heavier, than an equal volume of the wood 

 where the thin-walled tracheides with large lumina 

 predominate. 



Returning now to the section (Fig. 3), it is to be 

 observed that the differences in the zones just referred to 

 enable us to distinguish the so-called " annual rings." 

 The generally accepted explanation of this is somewhat 

 as follows. In the spring-time and early summer, the 

 cambium-cells begin to divide, and those on the inner 

 side of the cylinder of cambium gradually become con- 

 verted into tracheides (excepting at a few points where 

 the cells add to the medullary rays), and this change 

 occurs at a time when there is (i) very little pressure 

 exerted on the inner parts of the trunk by the cortex and 

 corky bark, and (2) only comparatively feeble supplies 

 are derived from the activity of the leaves and roots, in 

 the still cool weather and short days with little sunlight 

 In the late summer, however, when the thickened masi 

 of wood is compressed by the tightened jacket of elastic 

 bark which it has distended, and the Icng, hot, bright 

 sunny days are causing the numerous leaves and roots to 



