﻿Explanation of Plate LIV 



Fig. I. Cross-section of stem, Ptychoxylon Levyi, enlarged a little more than 

 one-half, a, outer zone of secondary wood, bordered with an 

 equally wide zone of rayed phloem, the whole being normal or 

 centrifugal in growth ; a and a", zones of inverted secondary wood 

 in bands temporarily continuous with the outer zone at the open- 

 ings where branches have broken through, one of the latter, r, 

 being still included in the section ; ai inner zone or cylinder of 

 inverted or centripetal wood with medullary rays, and rayed 

 phloem; r branch with secondary wood. In this specimen the pith 

 cells and primary wood are not shown, while the cortical tissues 

 were lost before petrification. From the Permian of Autun. 

 After Renault. 



Fig. 2. Cross-section of stem of Mcdullosa stcllata var. typica, slightly re- 

 duced. Small steles, each with its centrifugally developed sec- 

 ondary wood, are scattered in the central (principal) medullary 

 area; sha, outer, normal, secondary wood; shi, zone of inner and 

 inverted secondary wood, the medullary elements not appearing 

 in the illustration; Bi, zone of inner phloem or bast of the in- 

 verted exogenous wood ; the white line indicates the cambial zone ; 

 R, portion of cortex. From the Permian of Chemnitz. After 

 Weber and Sterzel. 



Fig. 3. Portion of the section of the interior of a trunk 48 cm. in diameter, 

 of Medullosa stcllata var. gigantea, about 9/10 the natural size. 

 Sm, ground mass of central pith containing a number of small 

 steles, st, with their radiate secondary wood, the primary ele- 

 ments being shown in the centers of some of the larger medullary 

 steles ; Shi, portion of inner zone of the enveloping cylinder of 

 inverted or centripetal secondary wood with its thick phloem, Bi. 

 Bi. 



FlG. 4. Cross-section of a fragment of the same specimen and just exter- 

 nal in position to that seen in figure 3. Shi, small fragment of 

 the inner, inverted, zone of secondary wood seen in the previous 

 figure; Pm, partial pith, including bundles, probably of primary- 

 wood ; Sha, normal or centrifugal secondary wood, with phloem, 

 B. and phloem rays; HiB, and H 2 B, succeeding zones of secondary 

 wood with rayed phloem zones. From the Permian of Chemnitz. 

 After Weber and Sterzel. 



