494 



SECONDARy CHANGES. 



element indi\-itluallv. In addition to those there are differences, to be discussed below, 

 which are presented by the structure of the same form of tissue in the spring and 

 autumn wood, and by the general grouping of all the elements of the ligneous body. 



I. In some few woods the bundles are composed of only one form of tissue, to the 

 exclusion of all the rest, and they then consist of Irachcides, which, apart from the 

 general differences between spring and autumn wood, have everywhere the same 

 structure. In these cases the parenchymatous system is represented only by the 

 medullary rays, which are inserted everywhere in great numbers between the bundles. 

 Among Dicotyledonous woods the Winterece belong to. this category; namely, Drimys 

 Winteri and its allies, Tasmannia aromatica\ and Trochodendron aralioides '^j the 

 systematic position of which is doubtful : among Conifercr, Taxus baccata belongs 

 here, according to Sanio, but in the case of this tree Hartig and Kraus state that scanty 

 bundle- parenchyma is present. 



In the other Conifers the Iracheidcs, of which the main mass of the wood is 



Fig. cog. — Juiiipenis comnumis; stem; cross-section throut^fh the autumn wood, bast, and cambium during ~ 



the winter's rest (end of September). 7i — /: outermost series of the autunm wood ; b^ b scries of Ijast-filires. At X 

 there is only one cambial cell between h and b ; 7n — in medullary rays. 



homogeneously composed, are accompanied by bundle-parenchyma, which sometimes 

 occurs in single vertical rows, scattered among the tracheides, wliile sometimes, as in 

 many AbietincK, it forms the coating of the resin- canals. 



In the Coniferae (Figs. 208 and 209) and the other plants jnst mentioned, the tra- 

 cheides are arranged in radial rows; they are quadrangular, as seen in cross-section, when 

 those which belong to neighbouring radial rows stand opposite to one another, hexagonal 

 or pentagonal when the radial rows are alternate: their ends are elongated and sharp, 

 owing to inclination of the radial surfaces (comp. p. 469). The radial surfaces have large 

 corresponding bordered pits, which in the Wintereae, the Araucariae, Dammarae, and the 

 wood of the root in other Coniferae, form two or more longitudinal rows, while in the 



. ' Goppcrt, Linnaea, Bd. XVI. p. 1.^,4.- -Kraus, I.e. 

 '' Eichler, in Flora, i'?-()\, p. 451. 



