XXIX] HETEUANGIUM 75 



The secondary xylem is continuous at its inner edge with the 

 outermost primary tracheids (fig. 411, A') and consists of rows of 

 tracheids, 1 — 3 elements broad, alternating with numerous broad 

 medullary rays of radially elongated parenchyma. Beyond a 

 typical cambium-zone the secondaryphloem consists of parenchyma 

 and sieve-tubes bounded by crushed arcs of primary phloem. 

 Abutting on the phloem is a pericycle composed of several layers 

 of small parenchymatous cells (fig. 411, A, f) and in the outer 

 layers of this tissue a pheUogen (fig. 411, C, p) and some periderm 

 are usually present though, as Williamson and Scott point out, 

 the periderm is less regular and narrower than in Lyginopteris. 

 The inner cortex, composed of short parenchymatous cells, is 

 traversed by numerous narrow bands of dark, thick-walled cells 

 similar in the structure of the elements, though pecuHar in the 

 horizontal elongation of the groups, to the sclerous nests in the 

 pericycle and pith of Lyginopteris.. These characteristic bands 

 are chiefly seen in the oblique longitudinal section of a stem 

 represented in fig. 412. In this section, 25 mm. in length, the 

 lighter band, p, is the pericycle and in it a few obliquely cut leaf- 

 traces are shown as dark patches. The horizontal bands are 

 similar in structure and shape to the diaphragms of thick cells 

 in the pith of Abies magnificat, and in both plants they probably 

 serve as supports to the softer parenchyma. There may be as 

 many as 46 bands in a vertical length of cortex of 1 inch (about 

 19 per centimetre). It was the occurrence of precisely similar 

 transverse Hnes on the carbonised impressions of the rachis of 

 Sphenopteris elegans that led KLidston^ to suggest a connexion 

 between that species and the stem of Heterangium Grievii. 



The outer cortex, consisting of alternate strands of parenchyma 

 and stereome similar to that of Lyginopteris, is much narrower and 

 a less conspicuous feature than in Heterangium; the stereome 

 bands do not form so regular a hypodermal network and extend 

 much further vertically without anastomosing. The epidermis 

 has been described as a layer of fairly thick cells showing in one 

 case an appearance of a depressed stoma*. There are no secretory 

 canals like those of Cycads but, as in Lyginopteris, scattered cells 



1 Jeffrey (05) PI. in. fig. 21. ° Kidston (9V) B. p. 49. 



' Williamson and Scott (95) p. 753. 



