PHYSIOLOGY OF TREES. 247 



takes place in every succeeding year during the 

 life of the tree ; the central axis or cylinder of 

 wood becoming annually enlarged in diameter by 

 the addition of a ring of alburnum, the whole 

 serving as a support to sustain and elevate the 

 branched head of the plant. 



Respecting these annual accretions of albur- 

 num^, a very important question forces itself 

 upon our notice, viz. Whence do these additions 

 originate? They have been already described in 

 both their conditions as cambium and alburnum; 

 but their origin has not been adverted to. In- 

 deed, of all the processes of vegetation, none is so 

 obscure as this. Its rise and progress into being 



*The annual rings of wood are very visible on a cross 

 section of the trunk. The number indicates the age of the 

 tree. In some kinds of trees, as the beech, for instance, the 

 alburnum, or sap-wood (as the timber-dealers call it), is per- 

 fect in the second year ; but the alburnum of oak is not 

 perfect till the sixth or seventh year. This is very obvious 

 on view of a section; six or seven of the outer layers will be 

 sap or white wood, and all the interior layers towards the pith 

 will be mature, as the colour slio^s. This fact also shows 

 that the new layer of oak formed this year (1829) will not be 

 perfect hearty timber till the year 1835. 



