SAPWOOD AND HEARTWOOD 337 



followed by the outgrowth of buds and the unfolding of 

 fresh leaves, will give rise to a double ring in one year, 

 so that the estimation of the age of a tree by counting 

 the rings is not always quite accurate. Sometimes a 

 similar annual alternation of bands of sieve tubes (spring) 

 and fibres (autumn) can be detected under the micro- 

 cope in the secondary phloem. 



Sapwood and Heartwood. When a tree-trunk has 

 reached a certain age, which varies in different species 

 of tree, the wood nearest the centre undergoes certain 

 changes, which gradually spread outwards as increase 

 in thickness continues. The living cells (rays and 

 xylem parenchyma) die and the walls of the lignified 

 cells lose water, increase in hardness and usually change 

 in colour, often becoming darker (oak), and sometimes 

 producing a pigment which stains them a distinct colour 

 (e.g. black in ebony, yellow in satin wood and yellow- 

 wood, purple in logwood) ; this harder internal wood 

 is called heartwood. It no longer functions as a water- 

 conducting tissue, and it is the wood most suitable for 

 use in building, furniture making, etc., because it does 

 not warp by losing much water through evaporation on 

 drying. The wood nearest the cambium which is still 

 conducting water and still contains living cells is called 

 sapwood. The sapwood forms a belt of approximately 

 constant width, continually added to on the outside by 

 the cambium, and continually converted into heartwood 

 on the inside. 



Some trees have no heartwood, the dead wood in the 

 centre remaining soft and often decaying after it dies, as 

 in the willow. This is frequently the cause of hollow 

 trunks, though hard heartwood too sometimes decays. 



Cork Formation. The phellogen or cork cambium 

 arises in most cases from the outermost layer of cortical 



22 



