380 



STRUCTURE OF PHANEROGAMIC PLANTS. 



FIG. 167. 





that the outer portion of each layer is almost exclusively composed 



of woody tissue ; such an arrange- 

 ment is seen in Fig. 165. This alter- 

 nation of ducts and woody fibre fre- 

 quently serves to mark the succes- 

 sion of layers, when, as is not un- 

 common, 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. There appears strong 

 reason to believe, 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 tempe- 

 rate climates, is usually (but 

 not invariably) a year, but 

 which is commonly much less 

 in the case of trees flourishing 

 in tropical regions. For ex- 

 ample, we not unfrequently 

 meet with stems, in which the 

 place of a layer of the ordinary 

 breadth is occupied by two nar- 

 row layers ; the line of demarcation between them having appa- 

 rently been formed by a temporary interruption to the process of 

 growth, in the middle of the period through which the formation 

 of wood extends. Such an interruption might occur from heat 



Transverse section of stem of Buckthorn 

 (R/iamnus). 



FIG. 108. 



Portion of the preceding figure, more highly 

 magnified. 



FlG. 169. 



Portion of transverse section of stem of Hazel, showing, in the portion a, &, c, six narrow layers of 



wood. 



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

 sphere, or from a fall of temperature in a tree that requires heat ; 

 and in a variable season, it might recur several times. Some- 

 thing of this kind would appear to have been the cause of the 



