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—OGUURNRSTYT IO WT i ee "WP TT WT T 
T6 DR. D. H. SCOTT ON THE HETERANGIUMS 
HĒ. shorense or even H. tiliwoides. Sometimes (as in specimen 5, Phor. 13) 
the amount of xylem-parenchyma is considerable. The peripheral xylem- 
groups are very distinct, the radial bands of parenchyma between them being 
continuous with the principal rays of the secondary wood (Pl. 2. Phots. 9, 12; 
Pl.4.fig.13). In this point the present species agrees with H. tiliwou/es and 
H. shorense *, and differs from the Lower Carboniferous M. Grievii, in which 
the peripheral strands are less isolated and the secondary wood not divided 
up by principal rays. 
Another feature of H. Lomaxii, which it shares with other Coal-Measure 
forms, is the position of the protoxylem, which lies very near the outside of 
the primary xylem, with only one or two centrifugal elements before the 
secondary wood is reached. "Thus the peripheral xylem-strands, though still 
mesarch, are approaching an exarch structure (see Phot. 10). This, again, 
is a marked difference from H. Grievii, where the centrifugal part of the 
primary wood is considerably more developed (see Williamson & Scott, 
1895, pl. 26. figs. 24, 25 t). The position of the protoxylem in 77. Lomari is 
well shown in longitudinal section in PI. 4, fig. 14, where the laxly coiled spiral 
element, which is no doubt the true protoxylem, is sueceeded to the outside 
by a single, more closely wound spiral tracheide, beyond which the wood 
appears to be secondary. On the inside the pitted elements are reached at 
once. Often, however, there is a little parenchyma on the inner side of the 
protoxylem. It appears that here, as in Lyyinopteris and Heterangium 
Grievii, the centrifugal part of the xylem was the first to be formed after the 
protoxylem. 
Elements with dark brown contents are very frequent in the xylem-paren- 
chyma, the rays, and throughout the stele. Whether they can be properly 
called “secretory sacs” is doubtful. They do not show, as a rule, any special 
elongation as compared with the associated cells. They are best shown in 
the rays, where they run in a radial direction ; here their contents are some- 
times in the form of a mass of granules, simulating altered starch-grains, 
but very variable in size. The * secretory " elements were mentioned in the 
joint memoir as characteristic of the Dulesgate specimens 1. 
The secondary wood varies greatly in amount, not only in the different 
specimens, but in different parts of the same specimen : it sometimes attains 
a very considerable development. Thus, in specimen 5, the thickness of the 
secondary wood ranges from 0°61 to 1:64 mm. (P1. 2. Phot. 13). In specimen 1 
the range is even greater, varying in one transverse section (S. 447 ; Phot. 9) 
from 0:3 to 2:58 mm. The excessive thickness at certain places is correlated 
with the presence of adventitious roots, as was already remarked in the joint 
* In the latter, the specimen is too young to show the relation to the principal rays. 
+ This distinction was noted iu the joint memoir, l. c. p. 750. 
1 Williamson & Scott, 1895, pp. 748, 792. 
