35] CHAPTER II. THE TISSUES 143 



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

 outer portion, the sap-wood (alburnum). 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 coloration 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 bast 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. 75, p. 96 ; 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- 

 chymatous cells with unlignified walls, corresponding to the thin- 

 walled fibrous cells of the secondary wood (p. 140), are sometimes 

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

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



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

 Abietinese, Fagus, Betula, Alnus, 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 Cupressinese and some 

 Taxoidese, 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, 

 Juglans, Sambucus, Rhamnus, Ulmus, Populus) ; or there may be 

 scattered groups of fibres (e.g. Cinchona, Morus, Larix, Celtis 

 Ficus elastica). 



The secondary bast does not, as a rule, attain so considerable a 

 size as the secondary wood, nor does it exhibit annual rings : this 

 is due to the fact that, except in some fleshy roots, it is formed in 

 smaller quantity, and further, to the fact that the older bast be- 



