162 



CONIFEBALES (RECENT) 



[CH. 



Sources of error in the detertnincUion of fossil Conifers. 



The determination of fossil Conifers is one of the most difficult 

 tasks of the palaeobotanist. It is comparatively seldom that well- 

 preserved cones are found in organic 

 connexion with the twigs that bore 

 them and the cones rarely exhibit those 

 features which are the best guides to 

 affinity. Excessive trust in superficial 

 similarity has frequently led to the em- 

 ployment of generic names suggesting 

 relationships which are thoroughly mis- 

 leading. In comparing fossil and recent 

 forms authors are apt to confine their 

 attention to the better-known types, 

 forgetting that it is often with the less 

 familiar and geographically restricted 

 genera that extinct plants are most 

 closely allied. Even the data supplied 

 by petrified wood are often insufficient 

 to enable the student to do more than 

 refer a specimen to some comprehen- 

 sive genus based on characters shared 

 by several recent genera. Though it 

 is as a rule easy to distinguish between 

 the wood of a Conifer and that of 

 Cycads and Dicotyledons, the agree- 

 ment between the xylem elements of 

 many Cycads and those of the Arau- 

 carineae is sufficiently close to afford 

 opportunity for error. The homo- 

 geneous structure of the secondary 

 wood of some Magnoliaceous genera, 

 e.g., Trochodendron and Dritnys^ (fig. 

 710), closely simulates that of a Conifer, 

 but the medullary rays are approxi- 

 mately equal in breadth to the tra- 

 cheids and the cells are more elongated 



^ Groppler (94) ; Solereder (99) p. 34; (08) p. 5. See also JefErey and Cole (16). 



Fir„710. Drimys W interi. Trans- 

 verse section of part of a stem. 

 p, pith ; /, pericycle fibres ; 

 c. cambium. 



