THE TISSUES OF THE STEM 



53 



As trunks grow old the colour and quality of the central wood changes in 

 many trees, but not in all. It becomes darker in colour, and harder, and it 

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

 durability, as distinct from the more superficial sap-wood, which is paler 

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

 when used in joinery. The change from heart-wood l^o sap-wood follows on 

 the death of the wood-parenchyma and medullary rays. The sap-wood, 

 as its name implies, is functional for conveyance of sap and for storage. The 

 heart-wood being dead serves only the purpose of mechanical support. It is, 

 however, liable to be attacked by certain fungi in the living tree, which bring 



FIG. 39. 



Wedge cut out of a four- year-old stem of Pine, in winter, q = transverse view ; 



Z=radial view; f = tangential view. /=spring, s=autumn wood. w=medulla. 



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



transverse, -ms', ms", in radial, ms'", in tangential view. c=cambium. 6=bast. 



1 6r=bark. ( x 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, 

 all that is otherwise necessary for normal life. 



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

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

 characteristic qualities to their wood and bast. Thin walls, and relatively 

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

 fibres grouped in solid masses give a hard wood, like that of the Oak or 

 Laburnum. Fibres may 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 tying up garden plants. The grouping among 

 themselves of the several tissues composing the wood and bast appears 



