Ett 
92 DR. D. H. SCOTT ON THE HETERANGIUMS 
On the other hand, we cannot be certain that the exarch tendency of the 
primary xylem-strands marks an advance, for we find both types of structure 
in species of similar age, as will be shown below. In the Coal-Measure 
forms, the centrifugal xylem, though little developed, is sharply differentiated 
and obviously primary ; indeed, from the nature of the traeheides (spiral or 
scalariform) it would appear that the centrifugal part of the strand was the 
first to be formed after the protoxylem, with which it is in direct connection, 
as in H. Grievii and Lyginopteris, a point on which Chodat has laid stress * 
(Chodat, 1908, p. 13). 
In the double leaf-trace and polydesmie petiole, the Coal-Measure species 
undoubtedly show a more complex organization than has hitherto been 
described in /Jeterangium, or even in Lyginopteris, for we have here to do 
with a trace which is double at its origin, not as in Lyginopteris with one 
that merely divides as it passes out. The further subdivision of the trace, 
giving rise to four or more bundles in the petiole, gives a most distinct 
character to the species in question ; the creation of a new genus for them 
might be suggested. There are, however, objections to this course, and I 
prefer to be content with proposing a provisional subgenus, which may be 
named PorvANGIUM f, and may best be characterized simply by the leaf-trace 
being double on starting from the stele, and dividing into four or more 
strands in the petiole. 
The proposed new subgenus will, of course, include the three species 
Hl. shorense, H. tilimoides, and H. Lomasi, with the probable addition of 
others. Æ. minimum, on the other hand, will fall under the other subgenus, 
which may be called Ev-HETERANCIUM, characterized by the single leaf-trace 
given off from the stele remaining single or dividing into two strands only 
on its passage outwards. Ihave not thought it desirable to introduce the 
other characters (distinctness of xylem-strands and tendency to exarchy ) 
into the diagnosis of the provisional subgenus, as we have no reason to 
assume that they are consistently correlated with the polydesmie condition. 
A rapid comparative survey of the other species of the genus and of some 
related groups will serve to make the position clear. 
CoMPARATIVE CONSIDERATIONS. 
Corda’s original species /Jeterangiwm paradoxum 1 was founded on fragmen- 
tary material, which practically shows nothing beyond the structure of the. 
metaxylem. Kubart (1911, p. 9, Taf. i. fig. 3), who examined smaller 
* Prof. Chodat uses the position of the protoxylem as an argument against the homology 
of the bundle in Lyginopteride with that in Cycadaceæ, a point with which we are not 
concerned in the present paper. 
+ Intended to suggest a polydesmie Heterangium. 
{ First published in 1845. The reference is to the edition of 1867: ‘ Flora der Vorwelt,’ 
p. 22, pl. xvi. 
