230 CORDAITEAE [CH. 



French leaves with those of Cordaites Felicis described by Prof. 

 Benson. 



The main features of Cordaites leaves are (i) the presence of 

 two kinds of xylem in the veins, the larger centripetal tracheids, 

 or chief water-conducting elements, and the narrower tracheids, 

 in some cases attached to the sides of the centripetal xylem, in 

 others forming free groups, usually between the protoxylem and 

 the phloem, but sometimes enclosing the phloem ; (ii) the frequent 

 presence of a well-defined sheath of cells round each vein composed 

 of comparatively thick-walled elements comparable with the 

 transfusion-tracheids in Conifers; (iii) the presence of lacunar 

 tissue in the centre of the mesophyll and in some cases of trans- 

 versely extended tracheids similar to those in some Podocarp 

 leaves ; (iv) a well-developed system of stereome-strands and 

 I-shaped girders. The structural features on the whole suggest 

 a xerophilous type, and the frequent absence or feeble develop- 

 ment of palisade tissue points to diffused rather than to brilliant 

 sunlight. 



The considerable range in size and form among Cordaitean 

 leaves as well as the obvious dependence on conditions of preserva- 

 tion or growth of such a relatively unimportant feature as the 

 presence or absence of the so-called false or interstitial veins 

 the variability of which has been demonstrated in several instances 

 renders specific determination exceedingly difficult. The follow- 

 ing species are briefly described rather with a view to illustrate 

 the nature of the characters employed by authors than as imply- 

 ing the existence of so many well-defined types. 



Cordaites principalis (Germar). 



This species was founded 1 on a large specimen from the Coal 

 Measures of Wettin showing a fan-like cluster of longitudinally 

 torn and partially overlapping leaves spread out in the position 

 that would be assumed on the compression of a shoot with a 

 close spiral phyllo taxis. This form of Cordaites is the most 

 abundant in the British Coal Measures. The broadly linear 

 lamina is characterised by an obtuse apex (fig. 466, A), a tendency 

 to split into strips, close-set parallel ribs, the stouter ribs or veins 



1 Germar (48) B. p. 55, PI. xxm. 



