64 DESCRIPTIVE CATALOGUE 



of the ray-cells; the presence and character of rny-trachcids 

 arid, in general, the degree of specialisation and differentiation 

 of the cells of the medullary rays. 



All the descriptions in the following catalogue of woods are 

 necessarily brief, and might be much elaborated if circumstances 

 permitted. For instance, the ray-tracheids in the Abietineee 

 have long interested botanists, but the descriptions of well- 

 preserved fossil ray-tracheids on pp. 98, 110, etc., refer merely to 

 the salient features which characterise the species. These fossils 

 would repay detailed examination, such as Thompson (1910) 

 has given to the ray-tracheids of living pines. All the phylo- 

 genetic conclusions reached by botanists from a study of living 

 species must bear critical examination in the light of the actual 

 features shown by these ancient fossil forms. Thus Thompson 

 (p. 14) says : " That they [the ray-tracheids] are of cenogenetic 

 origin is further indicated by the fact that in the older pines, 

 the Pityoxyla of the Cretaceous, as described by Jeffrey and 

 Chrysler, no ray-tracheids occur." [See also Holden (1913), 

 p. 62), liiiiley (1910, p. 293) and many others.] Xow Jeffrey 

 and Chrysler's Pityo.vyla are of Upper Cretaceous age, and in the 

 present work are described several forms of Lower Cretaceous 

 age in which beautifully developed ray-tracheids are present. 



It appears that for the present fossil woods are most usefully 

 described in relatively few "genera" with wide diagnoses, 

 and very minutely separated specifically, with very full and 

 careful specific diagnoses and descriptions. I therefore propose 

 to adopt the following scheme of classification : 



I. Resin-canals universally absent. Bordered pits ] 

 in two or more rows, alternating and liexagonallv 

 compressed, or in one row, flattened or rounded. 

 Medullary ray-cells all alike, with several small, 

 bordered pits per tracheid-field iu their radial 

 walls. J 



II. Resin-canals present or absent. Bordered pits 

 generally in a single row if in two, the pits 

 side by side and not alternating or compressed. 

 Medullary ray-cells sometimes highly specialised 

 and differentiated. Ray-tracheids often present. 



A. Secondary tracheids with strongly marked 1 



accessory spirals. j Taj-o.ryfon. 



