XXXII] CLADOXYLON 203 



secondary xylem varies considerably in the same specimen and 

 in places this tissue is hardly represented, a fact of importance 

 in view of the very striking resemblance between Arctopodium 

 and Cladoxylon, the sections referred by Unger to the former 

 genus having steles without any secondary xylem. The occur- 

 rence of one or two elongated spaces (shown in black in fig. 459, A) 

 near the distal end of each plate mark the position of the proto- 

 xylem tracheids. Fig. 459, C, represents a stele of a stem referred 

 by Unger and by Solms to Cladoxylon dubium which shows the 

 typical Cladoxylon structure, namely the central primary xylem 

 with distally placed protoxylem and the enclosing sheath of 

 secondary xylem. In the stem shown in fig. 459, D (C. dubium) 

 there are 12 steles, each constructed on the plan already described, 

 differing in their relatively broader and shorter form and in the 

 greater breadth of the secondary xylem from those seen in fig. 

 459, A (C. mirabile). The black areas in fig. 459, A, show the 

 primary xylem, and the protoxylem is seen in fig. 459, C. A stem 

 described by Dawson 1 as Asteropteris noveboracensis from Devonian 

 beds is compared by him with Unger's Cladoxylon mirabile and 

 regarded as possibly allied to it. The radial plates of xylem in 

 Dawson' s plant meet in the centre like those of Asterochlaena 

 and the leaf -traces are of the Clepsydropsoid type. 



The type of stem for which Unger founded his genus Schizoxylon 

 is represented in fig. 459, B ; there are five small steles in the 

 centre and external to these eleven radially arranged plates, 

 with oval steles between them, in the peripheral region of the 

 stem. Each stele consists of primary (black in the figure) and 

 secondary xylem and agrees with the steles in the other stems. 



From the type of stem illustrated by Cladoxylon mirabile to 

 that on which the genera Arctopodium 2 and Hierogramma 3 were 

 founded is a very small step: the vascular tissue has the same 

 characters both as regards gross and minute anatomy, but there 

 is no evidence of cambial activity in the stems referred to the 

 two latter genera, a difference in itself hardly worthy of generic 

 recognition. 



1 Dawson (81) A. p. 299, PL xn. figs. 19; Solms-Laubach (91) A. pp. 173, 188 



2 This type is represented in the Geological Survey Collection (No. 15871). 



3 For figures, see Solms-Laubach (96) B. PL n. 



