40 HBAETWOOD AND SAPWOOD. 



the internal is the liber. These two parts grow independently 

 of each other, by their inner faces ; the rind belonging exclu- 

 sively to the horizontal system, the liber composed of the 

 perpendicular and horizontal systems intermixed. 



In all Exogenous plants whose stems acquire an age beyond 

 that of a very few years, the wood is distinguishable into two 

 parts, heart-wood, and sap-wood or alburnum. The former is 

 more or less central, and coloured brown or some other tint ; 

 the latter is external, pale yeUow, and much softer. Heart- 

 wood was originally alburnum, and altered its nature with 

 age, in consequence of the solid matter with which all its 

 tubes and vessels were choked up ; alburnum is the youngest 

 wood, with all its communications free and open, no solid 

 matter having had time to accumulate within them. The 

 reason why solid matter collects in the tubes of wood, so as 

 gradually to choke them up, is this : the wood is the channel 

 through which all the fluid matter of a plant, whether crude 

 or digested, passes, in its way upwards to the leaves, or in 

 its horizontal direction from the bark to the central parts 

 of the stem. When sap leaves the earth and passes into the 

 stem, it ascends by the woody matter of the finest fibres of the 

 root : having left them, it flows into the new wood with which 

 those fibres are connected, and passes along this until it 

 reaches the leaves; on its return from them it descends 

 through the liber, in part passing off horizontally towards the 

 centre through the medullary rays. Wherever it passes it 

 deposits a portion of its solid parts ; and, consequently, that 

 portion of the wood, namely, the oldest or the heart-wood, 

 through which it has passed the most frequently, will have the 

 greatest quantity of matter accumulated within it, independently 

 of all other reasons for its hardening. 



In consequence of their peculiar manner of growth, new Kving matter 

 teing continually formed near the circumference of the trunk, and over 

 that which is older. Exogenous trees arrive at an old age wholly 

 unknown in the rest of the creation. And although some of the state- 

 ments on this subject may be exaggerations, yet as there is a certainty 

 that some individual trees have Uved for more than one thousand years, 

 so it is quite possible that others may have existed for a veiy much 



