XXXIIl] POROXYLON 219 



rows of contiguous alternate pits on the radial walls : the medullary 

 rays are 2 — 3 cells broad and may be 60 cells deep. According 

 to Eenault^ several small oblique> pits occur on the radial walls 

 of the ray cells. The secondary phloem, separated by a normal 

 cambium from the xylem, forms a broad band of sieve-tubes 

 with lateral sieve-plates like those in Medullosa anglica alternating 

 with tangential rows of parenchyma. The cortex is relatively 

 narrow and in older stems is chiefly occupied by secondary tissue 

 formed from deep-seated phellogens. 



Poroxylon Boysseti Renault. 



The stems of this species agree closely with those of P. 

 Edwardsii, the chief difference being in the structure of the 

 secondary phloem which does not show the regular concentric 

 alternation of sieve-tubes and parenchyma. 



Poroxylon stephanense Bertrand and Renault. 



This the oldest species, from Stephanian beds at Grand' Croix, 

 differs in no essential features from the other representatives of 

 the genus. It is from a study of the leaves of this type that 

 Bertrand and Renault have obtained most of the facts with 

 regard to the anatomy of Poroxylon fohage. In the median 

 region of the fleshy leaf the bundles are characterised by a com- 

 paratively large amount of centripetal xylem accompanied by 

 a considerable development of secondary centrifugal tracheids : 

 the bundles are connected laterally by both centripetal and 

 centrifugal xylem and thus at certain levels in the lamina the 

 vascular tissue has the form of a continuous plate (fig. 464, A, B). 

 The veins become independent on branching and near the edge 

 of the lamina they consist only of primary elements. Secretory 

 sacs of elongated form are scattered in the homogeneous meso- 

 phyll, and thick stereome-strands underhe the epidermis. The 

 epidermal cells are rectangular and rows of stomata occur on 

 both surfaces. 

 B. CORDAITEAE. 



Cordaites. Unger. 



A preliminary statement with regard to nomenclature may 

 serve to remove possible misconceptions in connexion with the 

 1 Renault (93) A. PI. Lxxw. fig. 8. 



