XLIV] GYMNOSPEEMOTJS WOOD 169 



shown, recent work has tended to reduce the taxonomic value of 

 certain characters such as the occurrence of ray-tracheids which 

 are more widely distributed than has generally been supposed. 

 ■In this connexion a word may be added with regard to some 

 common sources of error in anatomical investigation. There is 

 the obvious danger of confusion between features due to petrifying 

 agents or to decay before petrification and those present in the 

 living tree: the thickening of cell- walls, e.g., those of medullary 

 rays, has been shown in some cases to be a pathological pheno- 

 menon^. The partial obliteration of bordered pits by decay may 

 cause them to appear separate though originally in contact (fig. 

 475, B,Vol. HI. p. 257). The recognition of pits on the tangential 

 and horizontal walls of medullary-ray cells is often very difficult, and 

 negative evidence may be misleading. It is by no nieans always 

 a simple matter to distinguish between true canals and canal-like 

 spaces formed by the destruction of groups of tracheids {e.g., 

 Pityoxylon eiggense; fig. 725). In one case it has been shown 

 that leaf-traces traversing broad medullary rays were mistaken 

 for horizontal resin-canals^. The spiral lines frequently seen on 

 the walls of petrified tracheids caused by the directive influence on 

 the structure of the membrane of the course of enzyme-action may 

 simulate the spiral bands characteristic of Taxus, Torreya, and 

 Cephalotaxus. These are a few of the pitfalls in the path of the 

 palaeobotanist, but despite the difficulties and the frequency with 

 which imperfect preservation prohibits complete diagnosis, the 

 investigation of fossil wood is well worth the attention of students 

 equipped with an intimate knowledge of recent Conifers. The 

 unpromising nature of the material may be a deterrent, though 

 lignitic and other specimens not thoroughly petrified are amenable 

 to special treatment^. 



In the account of recent Conifers attention is called to the sig- 

 nificance of rings of growth : the subject has recently been exhaus- 

 tively treated by Antevs * and students should consult his memoir 

 in the Progressus rei botanicae for references to the literature. The 



1 Gothan (07), p. 25. 



2 Penhallow (00) p. 76 ; Thomson and Allin (12). 



'' For methods, see Jeffrey and Chrysler (06); Hollick and Jeffrey (09) B. 

 Gothan (09); Sinnott (09). 

 * Antevs (17). 



