268 PHYSIOLOGY OF TREES. 



the last year's alburnum so entirely, that its 

 existence as a distinct membrane cannot be 

 doubted : no disruption or dilatation, as before 

 observed, takes place, except where the medullary 

 rays pass from the bark into the wood. 



The liber of the seventh year is also formed in 

 the summer. It is a sloug-h, thrown off from 

 the swelHng cambium, but quite distinct, and is 

 no more connected with the cambium at the end 

 of the growing season than the cambium is with 

 the last year's wood, except only by the con- 

 verging rays which seem to connect together 

 the whole structure of the stem. 



The dotted space 7 (fig. B), is the increasing 

 cambium, or new layer of wood, of the seventh 

 year, which only occupied the line 8 during the 

 previous winter, and will be confined to a 

 similar line on the exterior of the dotted space 

 during the next. Hence it will be observable 

 that the vital principle shifts its place every year. 

 The alburnum which by its agency was distended 

 into complete form last year, is deserted by it at 

 the beginning of this ; but whether dormant, as 

 it was on the line 8, or actively swelling, as in 



