PHYSIOLOGY OP 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 shows. This fact also shows 

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

 perfect hearty timber till the year 1835. 



