XXXII] PTYCHOXYLON 189 



have 3 — 5 rows of bordered pits on the radial walls. At intervals 

 the continuity of the main stele is broken by the formation of 

 leaf-gaps and before one gap is repaired a second may be produced, 

 thus converting the cylinder into two crescentic and infolded 

 bands (fig. 453, A). A striking character is the occurrence in the 

 large parenchymatous central region of internal vascular bands 

 or arcs varying in size and number at different levels and com- 

 posed of centripetally developed secondary xylem and phloem. 

 These internal bands differ from the outer and broader cylinder 

 both in their inverse orientation and their hmited vertical range. 

 The connexion between the inner and outer vascular tissue and 

 the alteration in plan of the conducting tissue at different levels 

 are illustrated by fig. 453, 1 — 4, simphfied from some of Renault's 

 figures of successive sections through a vertical distance of 4 — 5 cm. 

 In section 1 the main cylinder is continuous except for a small 

 gap where a leaf -trace is about to be given off: there are three 

 internal vascular bands similar in structure to the outer stele 

 but inversely orientated. At a higher level (section 2) the leaf-gap 

 is larger and in it is a double leaf-trace of two collateral strands 

 consisting of primary centripetal xylem and a fan-like group of 

 secondary xylem and phloem. The free edges of the outer stele 

 of section 1 have curved inwards and united with the two lateral 

 medullary bands, while the lower internal band of section 1 has 

 increased in extent and forms a discontinuous arc with the upper 

 portions enclosed by the loops formed by the infolded ends of 

 the outer vascular tissue. In section 3 a second leaf-gap has 

 been formed in the outer stele and its invaginated ends have 

 fused with the internal bands. In section 4 the first leaf-gap is 

 closed and the invaginated bands of section 3 have broken up 

 into an irregular circle of shorter bands. The section reproduced 

 in fig. 453, A, shows the main cyhnder in the form of two curved 

 and flattened loops, each composed partly of the centrifugally 

 developed xylem and phloem of the main stele and in part of the 

 inversely orientated tissue of the inner bands. At a lower level 

 the two bands b, b, will become detached as the upper leaf-gap 

 is closed and form part of an inner cyhnder like the discontinuous 

 elhpse formed by the two bands c. The section of a branch-stele 

 is seen at a. 



