134 



CONIFERALES (RECENT) 



[CH. 



Araucarian pits as small (9 12-8 ju) in contrast to the broader pits 

 (up to 21ft) of the Abietineae, those in Cupressineae being inter- 

 mediate in size. There may, however, be considerable difference 

 in the size of the pits in a single type 1 . The occurrence of spiral 

 thickening bands in addition to bordered pits is characteristic of 

 the Taxineae, but spiral bands occur sporadically in the secondary 

 tracheids of other Conifers, e.g., Phyllocladus, Larix leptolepis, 

 species of Abies and other Abietineae 2 , also in some species of 

 Cupressus 3 . In Pseudotsuga spiral bands may occur in all the tra- 

 cheids of an annual ring. Some authors assert that the arrangement 

 and grouping of the bands in a tracheid constitute a character of 

 generic value, but there is not complete agreement on this point 4 . 

 The walls of tracheids frequently exhibit well marked spiral 

 patterns 5 , due to an entirely different cause, which, especially 

 in some petrified woods, closely simulate spiral bands. In the 

 process of decay enzyme-action may etch into prominence the 

 striation or spiral method of wall-construction; but the spirals 

 are steeper than those of the true thickening bands. The presence 

 of xylem-parenchyma, though of diagnostic value, is too uncertain 

 and variable a character to be used with great confidence. In 

 young shoots of Sequoia xylem-parenchyma may be absent though 

 it is present in older branches 6 . Such parenchyma occasionally 

 occurs in Abietineoiis wood 7 , but it is generally considered a charac- 

 teristic feature of the Cupressineae though in the wood of some 

 members of that family it is not always obvious. The presence 

 of drops of resin in the cells which form vertical series in different 

 parts of the wood, or only in the late summer wood, may render 

 the xylem-parenchyma conspicuous both in transverse and longi- 

 tudinal sections. Rows of parenchyma occur in the wood of 

 Abies pectinata 8 , also in Podocarpus and Dacrydium. In Taxodium 9 

 the thick horizontal walls of the cells are a characteristic feature. 

 Wood-parenchyma is raie in the Araucarineae and, as Penhallow 10 



1 Schenk in Schimper and Schenkt90) A. p. 848. 



^o,,^ vv ,. 3 Jones (12); (13). 



Gothan (05) p. 54; Penhallow (07) p. 41 ; Burgerstein (08) p. 104; Kraus (83) 

 p. 103; Nakamura (83). 



2 Bailey (09). 



4 



5 Kraus (88); Gothan (05). 



7 Burgerstein (06) ; Bailey (09). 



9 Schroeter (80) p. 30. 



6 Conwentz (92) p. 35. 

 8 Kny (10). 

 10 Penhallow (04). 



