35.] CHAPTER II. THE TISSUES. 199 



ring was found to be 15 mm. in thickness : here the fibres had thinner walls, 

 and the vessels, though more uniform in size, were not quite so wide as in the 

 narrower ring previously described. 



The annual ring is by no means always of equal thickness all round : it is 

 frequently thicker on one side of a stem or of a root than the other, so that the 

 genera] structure is strongly excentric. 



The secondary wood gradually becomes distinguishable into an 

 older internal portion, the heart-wood (duramen), and a younger 

 outer portion, the sap-wood (alburnuni). This arises from, the fact 

 that, as the wood becomes older, the cells of the wood-parenchyma 

 and the fibrous cells die and lose their protoplasmic cell-contents ; 

 as a consequence, the heart- wood has less water in its composition 

 than the sap-wood. In some cases this change is accompanied by 

 a colouration of the cell- walls of the heart- wood, with the result 

 that the distinction of duramen and alburnum is most marked 

 (e.g. Pine, Larch, Oak) ; it is but rarely that this distinction is not 

 observable (e.g. Buxus, Acer pseudoplatanus and platanoides). 



The structure of the secondary last essentially resembles that of 

 the primary bast. It always consists of sieve-tubes and of paren- 

 chyma, and very frequently of thick- walled fibres as well. 



The sieve-tubes of the secondary bast have the compound sieve- 

 plates shown in Fig. 98, p. 137 ; in Dicotyledons they have com- 

 panion-cells developed in relation with them. The parenchyma 

 very much resembles that of the secondary wood, except that its 

 cell-walls are not lignified ; it is abundantly developed in certain 

 fleshy roots (e.g. Taraxacum, Rubia, and the Carrot and Parsnip), 

 where it constitutes the chief part of the secondary bast. Prosen- 

 chyniatous cells with unlignified walls, corresponding to the thin- 

 walled fibrous cells of the secondary wood (p. 196), are sometimes 

 present. The bast-fibres closely resemble the woody fibres, but 

 their walls are not lignified (Fig. 148 G). 



In many cases the secondary bast contains no bast-fibres (e.g. 

 Abietineae, Fagus, Betula, Alrius, Platanus, Cornus, Ephedra, etc.). 

 When, as is usually the case, bast-fibres are present, their arrange- 

 ment presents considerable variety : there may be alternating tan- 

 gential layers of fibres (hard bast) and of sieve-tubes and paren- 

 chyma (soft bast), as in the case of the Cupressineae and some 

 Taxoideae, and, though with less regularity, in many Dicotyledons 

 (e.g. Vitis, Spiraea, species of Acer, Tilia, species of Salix, etc.) ; 

 more commonly the tangential layers of fibres are interrupted here 

 and there by soft bast (e.g. Quercus, Corylus, Carpinus, Pyrus, 



