370 THE MICROSCOPE AND ITS REVELATIONS. 



appearance to the transverse section (Figs. 254, 255). The general fact, 

 however, is, that the ducts predominate towards the inner side of the 

 ring (which is the part of it first formed), and that the outer portion of 

 each layer is almost exclusively composed of woody tissue: such an arrange- 

 ment is shown in Fig. 253. This alternation of ducts and woody fibre- 

 frequently serves to mark the succession of layers, when, as it is not un- 

 common, there is no very distinct line of separation between them. 



369. The number of layers is usually considered to correspond with 

 that of the years during which the stem or branch has been growing; and 

 this is, no doubt, generally true in regard to the trees of temparate cli- 

 mates, which thus ordinarily increase by 'annual layers.' There can be 

 no doubt, however, that such is not the universal rule; and that we should 

 be more correct in stating that each layer indicates an epoch of vegetation; 

 which, in temperate climates, is usually (but not invariably) a year, but 

 which is commonly much less in the case of trees nourishing in tropical 

 regions. Thus among the latter it is very common to find the leaves, 

 regularly shed and replaced twice or even thrice in a year, or five times in 

 two years; and for every crop of leaves there will be a corresponding layer 

 of wood. It sometimes happens, even in temperate climates, that trees 

 shed their leaves prematurely in consequence of continued drought, and 

 that, if rain then follow, afresh crop of leaves appears in the same season; 

 and it cannot be doubted that in such a year there would be two rings of 



Portion of Transverse Section of Stem of Hazel, showing, in the portion a, 6, c, six narrow layers 

 of Wood. 



wood produced, which would probably not together exceed the ordinary 

 single layer in thickness. That such a division may even occur as a conse- 

 quence of an interruption to the processes of vegetation, produced by sea- 

 sonal changes, as by heat and drought in a tree that flourishes best in a 

 cold damp atmosphere, or by a fall ol temperature in a tree that requires 

 heat, would appear from the frequency with which a double or even a 

 multiple succession is found in transverse sections of wood to occupy the 

 place of a single one. Thus in a section of Hazel stem (in the Author's 

 possession), of which a portion is represented in Fig. 256, between two 

 layers of the ordinary thickness there intervenes a band whose breadth is 

 altogether less than that of either of them, and which is yet composed of 

 no fewer than six layers, four of them (c) being very narrow, and each 

 of the other two (a, b) being about as wide as these four together. The 

 inner layers of wood, being not only the oldest, but the most solidified 

 by matters deposited within their component cells and vessels, are spoken 

 of collectively under the designation duramen or ' heart- wood.' On the 

 other hand, it is through the cells and ducts of the outer and newer lay- 

 ers that the sap rises from the roots towards the leaves; and these are 

 consequently designated as alburnum or * sap-wood.' The line of demar- 

 cation between the two is sometimes very distinct, as in Lignum-vitae and 

 Cocos wood; and as a new layer is added every year to the exterior of the 



