OF THE BRITISH COAL MEASURES. T 
memoir of 1895*. In other specimens the secondary growth has made little 
progress. Thus in a section (W. 1915 P) referred to specimen 3, the thick- 
ness ranges from *08 to *3 mm. only, and in specimen 2 it is about the same 
(see Pl. 4. fig. 13). These relatively voung specimens are of interest for com- 
parison with the unthickened stems included under our form «æ (see page 85). 
As is usual in //eterangium, the first-formed elements of the secondary 
wood are considerably smaller than the primary metaxylem tracheides ; 
further out, if the growth is continued, they generally become larger, 
though seldom reaching the size of the largest primary tracheides (see Pl. 2. 
Phots. 9 and 13). Local tangential bands of exceptionally narrow tracheides 
are frequent. 
The structure of the secondary wood is like that of other species of 
Heterangium. The tracheides have the usual multiseriate bordered pits on 
their radial walls. Where, however, the elements are very narrow, the 
number of rows may be reduced to two or even, locally, to one. Where the 
pits are well preserved, we see that the border has a narrow inclined slit. 
In places the pits show traces of a spiral arrangement, but the definite 
spiral bands noticed in H. tiliwoides have not been observed here. 
As I have seen no good tangential section it is difficult to say anything 
definite about the structure of the tangential walls. Certainly the pits are 
very few here compared with those on the radial walls, but there is evidence 
for the occurrence of scattered tangential pits f. 
The principal rays—those corresponding to the intervals between the 
primary xylem-strands—are of considerable width, amounting to 9 or 10 
cells in the larger specimens. The secondary rays vary much in width and 
in some cases appear to be uniseriate. The principal rays are occasionally 
divided up by intercalated tracheides, at some distance out, and, conversely, 
new rays appear as the wood increases in thickness. The ray-cells are thin- 
walled and much elongated in the radial direction. 
'The phloem is fairly preserved in one case only, specimen 5 (see Phot. 13). 
It here attains a maximum thickness of about 0*8 mm. I have not found such 
marked dilation of the phloem-rays as characterizes H. tiliwoides. The 
secondary phloem consists for the most part of fairly large, radially arranged 
elements. A small-celled irregular tissue sometimes preserved on the outer 
border is no doubt the primary phloem. Ii the absence of satisfactory 
longitudinal sections no details of the phloem-structure can be given. 
The pericycle forms a broad zone, and is often better preserved than the 
cortex, as in H. tiliwoides (Phots. 9, 12, 13). Its normal width, apart from 
bulges where bundles or roots pass out, is about 0'6 mm. in specimens 1 
and 6, where it is best shown. 
* Williamson & Scott, 1895, p. 757. 
+ Cf. Williamson & Scott, 1895, p. 752. 
