﻿white] fossil plants of group cycadofilices 379 



tantly anastomosing. When numerous the smaller arc mostly cen- 

 tral, and the larger, more or less ribbon-like and dilated, are con- 

 centrically disposed near the periphery. Each stele contains a cen- 

 tral axis of parenchymatous conjunctive tissue traversed throughout 

 by anastomosing groups of primary strands. The larger groups, 

 which are peripheral, include pitted tracheides, and are exarch. 1 

 Each stele is provided with a well-developed zone of secondary wood, 

 loose and spongy in texture, with wide and high medullary rays 

 (see plate li\ , fig. 2). The tracheides are multiserately punctate with 

 bordered pits on their radial walls. The secondary wood is followed 

 by cambium, phloem with phloem rays, and pericycle, while the entire 

 group of steles is enveloped in a periderm surrounded by cortical 

 short-celled parenchyma containing secretory canals suggestive of 

 the gum canals of Cycads. In one species the outer steles are greatly 

 broadened on the peripheral side so as practically to coalesce in a 

 continuous zone or cylinder of normal secondary wood, the second- 

 ary wood of the inner side of the steles being inverted with reference- 

 to the whole trunk and often forming an inner cylinder of inward- 

 growing exogenous wood. In another form (plate liv, figs. 3 and 

 4) the entire stelic zone is surrounded by several successively devel- 

 oped extra-fascicular zones or layers of wood and phloem, an anom- 

 alous development which has an analogy in the living Cycas and 

 Encephalartos also. 



The leaf traces of Medullosa consist at first of both primary and 

 secondary wood, each derived from its own kind on the peripheral 

 side of the stele, in concentric arrangement ; but in passing outward 

 the secondary wood in most cases is lost, and the primary wood 

 separates, while the pitted tracheids disappear, into many small col- 

 lateral strands, the protoxylem being outward, next the phloem. 

 Several leaf traces originating at different levels furnish large num- 

 bers of strands to a single petiole. 



The petioles of Medullosa, before their final correlation with the 

 parent stems, had been described as Myeloxylon and Myelopteris. 2 

 They comprise a ground mass of short-celled tissue including secre- 



1 In describing the arrangement of the woody elements in a bundle col- 

 lateral in structure the terms "exarch," "mesafch" and " endarch " are used 

 by Williamson and Scott according to the position of the primitive spiral 

 trachea? (protoxylem) at the outer border of the bundle, in the interior of 

 the bundle, or at the inner border of the bundle. 



2 See Renault, Mem. Sav. etr. Acad. Set., vol. xxn, no. 10, 1876; Renault, 

 Cours dc botaniquc fossile, vol m, 1883, p. 165; Zeiller, Fl. Foss. bassin 

 houill. et perm. d. Autun et d'Epinac, pt. 1, Paris, 1890, p. 282; Zeiller, Ele- 

 ments dc Palcobotaniquc, Paris, 1900, p. 131. 



