SECT. I 



MORPHOLOGY 



143 



stem, the fewer will be the annual rings. As the diagram in Fig. 150 

 shows, the older rings disappear first on passing towards the apex, 

 and in the same way the older layers of the bast are unrepresented 

 nearer the apex. Up to a certain period, in the age of woody plants, the 

 elements of both wood and bast exhibit a progressive increase in size. 

 The living elements may remain in a state of greater or less 

 activity throughout the whole of the wood, extending even to the 

 pith ; such wood is called splint wood : the Beech (Fagus sylvatica) 

 may be quoted as an example. In other trees which form heart-wood, 



m 



Fig. 151. — A, Transverse section of the wood of a Pine at the junction of two annual rings. /, iSpring 

 wood ; s, autumn wood ; t, bordered pit ; a, interposition of a new row of traeheides resulting 

 from the radial division of a cambium cell ; h, resin canals ; in, medullary rays ; g, limit of 

 autumn wood, (x 240.) B, Part of a trans\erse section of the stem of a Pine, s, Late wood ; 

 c, cambium ; p, sieve-tubes ; j), bast parenchyma ; k, cell of bast parenchyma containing crystal; 

 CO, sieve-tubes, compressed and functionless ; m, medullary ray. (x 240.) 



the living elements die after a certain time, so that only dead tissues 

 are found at a certain distance from the cambium. Before the death 

 of the living cells, they usually produce certain substances, such as 

 tannin and gums, which permeate the cell walls of the surrounding 

 elements, and also partially close their cavities. The tannins impart 

 to the dead wood a distinct colour, often very characteristic, especially 

 when it has been transformed into "wood dyes, or so-called xylochrome. 

 The tannin in the woody walls acts as a preservative against decay, 

 while the gums close the functionless water-courses of the dead wood. 

 The dead portion of the wood of a stem is called the HEART-WOOD or 

 DURAMEN, in contrast to the l-ving SAP-WOOD or ALBURNUM. Usually 



