408 THE WOOD. PART n. 



new ring- of young sap-wood, exterior to all its pre- 

 decessors. A portion of the cambium, in its unchanged 

 or liquid state, always remains between the wood and 

 the bark, which are never in absolute contact. 



As a new cylinder of wood enclosing all its predeces- 

 sors is annually formed, the section of a stem perpen- 

 dicular to its axis exhibits a ring of woody fibre, 

 alternating with a ring of spotted and rayed vascular 

 tubes, which constitute the silver grain of the wood. 

 The rings are more and more crowded, and narrower 

 towards the centre, and at last become impervious to 

 the sap, which only rises through the younger part of 

 the sap wood. In fact, a large portion of the solid 

 fibres of most plants have ceased to take any active 

 share in the performance of vital functions, and, like 

 the solid heart of an oak, retain their integrity simply 

 because they are not exposed to influences which 

 would cause their decomposition. A vegetable tissue 

 exposed to ordinary chemical action, can only remain 

 entire so long as it is performing vital functions. The 

 arrangement of the woody fibre and ducts in the differ- 

 ent orders and genera is much varied. The breadth of 

 the rings of wood shows the effect of good and bad 

 seasons ; and in extra-tropical latitudes, where there is 

 alternately a period of growth and repose, their num- 

 ber frequently indicates the number of years' growth ; so 

 that the age of a tree may be approximately, if not ex- 

 actly, learned from a critical examination of a section 

 of its stem. 



The innermost cylinder of wood is lined by the medul- 

 lary canal or tube containing the pith. It is a delicate 

 membrane, entirely formed of hollow spiral tubes. The 

 pith, which fills the canal, is of greenish cellular tissue 

 when young, full of sap, and occasionally, though rarely, 

 mixed with vascular and spiral tissue. It passes un- 

 interruptedly to the end of every branch, leaf bud, and 



