140 WorsdelL—The Structure and Origin of the Cycadaceae. 
or concentric bundle , and is hence precisely comparable to, and homologous 
with, each bundle in the peduncle of Stangeria , &c. ; both, in my opinion, 
have been derived phylogenetically from a single stele, like one of those 
composing the cylinder of Medullosa. The following are my reasons for 
holding this isolated position. In many stems of Lyginodendron the 
bundles are more or less separate from one another, or in groups of two, 
due to lateral anastomosis ; each single or double group of primary xylem 
having an arc- or fan-shaped mass of secondary xylem attached to it 
(Fig. 6) ; this in itself is an indication of the stelar origin of the bundle ; 
otherwise, how is this arc-like character to be explained ? 
The fact that arc-shaped leaf-trace bundles are cut off from the cylinder 
(which may occasionally be concentric, thus resembling the case of Medullosa) 
will not explain the phenomenon ; for similar arc-shaped leaf-traces pass off 
from the stele of Heterangium ; the arc-shaped character is one which is 
inherent in the stem-bundles as such. 
Fig. 6. Lyginodendron Oldhamium : transverse section of vascular ring of stem of young 
plant, It = leaf-trace (diagrammatic) (after Williamson and Scott). Fig. 7. Transverse 
section of single bundle from vascular ring of stem, showing its concentric stele-like structure, the 
remaining bundles of ring had same structure (from slide no. 1138 of Williamson Collection in 
British Museum) (diagrammatic). 
In some stems, however, where the bundles are more isolated, the 
cambium of the dorsal side extends right round the centripetal xylem on 
the ventral side (Fig. 7), though its secondary products in this region, as 
might be expected, are much less in amount, the parenchymatous often 
outnumbering the xylem-elements. This phenomenon strongly supports 
my position as to the polystelic origin of the Lyginodendron- cylinder. 
Scott, on the other hand, maintains that they do not in the least disturb 
his position as to the monostelic origin of the latter ; for these extra tissues, 
giving rise to the concentric structure, are purely secondary , formed by 
a cambium which, in his opinion, may appear on the scene at any time 
and under any circumstances , and to which, therefore, no morphological 
significance is to be attached. He regards it as the same anomaly as 
occurs in Acanthophyllum (Caryophyllaceae), in lodes tomentella ( 01 a- 
cineae), and in Tecoma (Bignoniaceae), and as far as the bare facts are 
concerned, these latter cases are, doubtless, comparable with that of 
