THE TISSUES OF THE STE;\I 



53 



As trunks grow old the colour and quality of the central ^\ood changes in 

 man}' trees, but not in all. It becomes darker in colour, and harder, and it 

 is distinguished as heart-wood. It is prized bv joiners for its strength and 

 durability, as distinct from the more superficial sap-'d;ood, %vhich is paler 

 in colour, softer in texture, and more hable to the attacks of vermin, or fungi, 

 when used in joinery. The change from heart-wood to sap-wood follows on 

 the death of the wood-parench-\-ma and medullary rays. The sap-wood, 

 as its name implies, is functional for con^'eyance of sap and for storage, The 

 heart-w-ood being dead ser\'es only the purpose of mechanical support. It is, 

 however, Uable to be attacked by certain fungi in the li^-ing tree, which bring 



Wedge cut out of a four-vear-old stem of Pine, iu \\'iater. ^^transverse ^new ; 

 /=radial \'iew; (=tangeiitial \ie\v. /=spring, 5 = autuiiLD wood. 7n = medulla. 

 i>=protoxylem. i, 2, 3, 4=successive annual rings. Jji5 = medullary ray in 

 transverse, )ji-s', his", in radial, ms'", La tangential view. c=carabium. 6=bast. 

 6r=bark. { >: 6.) S. 



about its decay. Such hollow trees, though mechanically weakened, retain 

 in their external sap-wood and cambium, and in the more superficial tissues, 

 aU that is otherivise necessary' for normal Ufe. 



The occurrence and proportion, as well as the mutual arrangement of the 

 component tissues are variable in difierent stems. It is this that gives the 

 characteristic quaUties to their w-ood and bast. Thin walls, and relatively 

 few fibres result in soft wood, as in the Lime. Thicker waUs, and numerous 

 fibres grouped in soUd masses give a hard wood, hke that of the Oak or 

 Laburnum. Fibres mav be absent from the bast, as in the Currant ; or they 

 may be present in large numbers, forming irregular masses, as in the Lime, 

 which gives the " bast " for tjing up garden plants. The grouping among 

 themselves of the several tissues composing the wood and bast appears 



