STRUCTURE OF DICOTYLEDONOUS STE31. 409 



various kinds ; these, however, are absent in one large group, the 

 Coniferce or $ Fir Tribe with its allies (Plate xm. , fig. 4, and 

 Figs. 215-217), in which the "Woody Fibres are of unusually 

 large diameter, and have the peculiar Glandular markings already 

 described (§ 294). In any Stem or Branch of more than one year's 

 growth, the Woody structure presents a more or less distinct ap- 

 pearance of division into Concentric Rings, the number of which varies 

 with the age of the tree (Plate xn., fig. 2). The composition of the 

 several Rings, which are the sections of so many cylindrical Layers, 

 is uniformly the same, however different their thickness ; but the 

 arrangement of the two principal elements, — namely, the Woody 

 fibre and the Ducts, — varies in different species : the Ducts being 

 sometimes almost uniformly diffused through the whole layer, but 

 in other instances being confined to its inner part ; while in other 

 cases, again, they are dispersed with a certain regular irregularity 

 (if such an expression may be allowed), so as to give a curiously- 

 figurecl appearance to the Transverse Section (Plate XII., figs. 2, 3). 

 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 arrangement is shown in Plate xn., fig. 1. 

 This alternation of Ducts and Woody fibre frequently serves to 

 mark the succession of Layers, when, as it is not uncommon, there 

 is no very distinct line of separation between them. — 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 Temperate climates, 

 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 flourishing in Tropica><egions. Thus among the latter it is 

 very common to find the Leaves regularly shed and replaced twice 

 or even thrice in a yeai*, or Jive 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, a fresh crop of leaves appears in the same 

 season ; and it cannot be doubted that in such a year there would 

 be two rings of Wood produced, which would probably not together 

 exceed the ordinary single layer in thickness. That such a division 

 may even occur as a consequence of an interruption to the processes 

 of Vegetation produced by seasonal changes, — as by heat and 

 drought in a tree that flourishes best in a cold damp atmo- 

 sphere, or by a fall of temperature in a tree that requires heat, — 

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

 multiple succession of Rings is found in Transverse Sections of wood 



