704 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS 



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 indi- 

 cates an ' epoch of vegetation,' which, in temperate climates, is usually 

 (but riot 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, 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 



FIG. 544. Portion of transverse section of stem of hazel, showing, in the portion 

 a, b, c, six narrow layers of wood. 



in a tree that flourishes best in a cold, clamp atmosphere, or by a fall 

 of temperature in a tree that requires heat would appeal- from the 

 frequency with which a double or even a multiple succession of rings 

 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 posses- 

 sion), of which a portion is represented in fig. 544, 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 rings of wood, being not only the 

 oldest, but the most solidified by resinous 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 layers that the 

 sap rises from the roots towards the leaves ; and these are conse- 

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

 cation between the two is sometimes very distinct, as in lignum vitse 

 and cocos-wood ; and as a new ring is added every year to the ex- 

 terior of the alburnum, an additional ring of the innermost part of 

 the alburnum is every year consolidated by internal deposit, arid is 



