64 



BOTANY OF THE LIVING PLANT 



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 sap-wood to heart- 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, 



I* ~h 



Fig. 42. 



Wedge cut out of a four-year-old stem of Pine, in winter. Though the Pine is a Gymno- 

 sperm, the construction of its trunk is on the same plan as that seen in Angiosperms. 

 q= transverse view; /= radial view; t = tangential view. /=spring,s = autumn wood. m= 

 medulla. £ = protoxylem. h, h = resin-passages. 1, 2, 3, 4 = successive annual rings. ms= 

 medullary ray in transverse, ms', ms", in radial, ms", in tangential view. c= cambium. 

 fe=bast. 6r=bark. ( x 6.) Strasburger. 



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

 about its decay. Trees hollowed by such means, though mechanically 

 weakened, retain their external sap-wood and cambium, and so possess 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 



