656 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
characteristic. No trace of any scalariform or pitted tracheides has been seen. The 
tracheides were of considerable length, and had pointed ends. 
The leaf-traces arise from the arms of the xylem, more than one vertical series 
coming from each arm, at least in the case, of the large and medium-sized stems. 
This is shown in a number of figures, but the reader may be especially referred to the 
steles in figs. 53 and 57. The arrangement and relations of the leaf-traces to the stem- 
xylem are also well shown in figs. 55 and 56, in which the tissues other than the xylem 
are partially or wholly omitted owing to the screen used in taking the photograph. 
The xylem of the larger leaf-traces, which often arose from steles in which the 
protoxylem was recognisable, shows a clear differentiation into small central 
tracheides (protoxylem) surrounded by larger metaxylem elements (figs. 80 and 8l). 
These in suitable specimens could be seen to be respectively continuous with the 
protoxylem and metaxylem of the stem. The differentiation may be evident in the 
leaf-trace even when it cannot be traced into the stem xylem (fig. 82). It can be 
seen even in small traces, although in them the metaxylem elements often form 
an incomplete layer around the protoxylem. 
The leaf-traces exhibit a considerable range in size which does not go strictly 
parallel with the size of the stems or steles. They are often larger, and sometimes 
hollow near the periphery of the stem, an appearance that is probably in part to be 
accounted for by changes occurring with age (fig. 43). 
• The xylem of the leaf- trace acquires a sheath of the soft tissues or phloem which 
surrounds the xylem of the stele. This appears according to the preservation 
as composed of more or less collapsed or decayed, elongated cells. The nature of 
this tissue in the leaf-trace and its continuity with the phloem of the stem is most 
clearly seen when the latter tissue is well preserved, as in the specimen from the 
transition region already described, which is represented in figs. 19 and 21. 
The leaf-traces thus arising from the stele seem to have pursued an almost vertical 
course for some distance, and are therefore commonly met with in the immediate 
neighbourhood of the stele, and especially in the outer zone of phloem. This is seen 
in many of the transverse sections, and also in fig. 63. The leaf-trace seems to have 
been more oblique in its passage through the inner cortex, but on reaching the outer 
cortex it is again almost vertical, till it ends in the leaf-base. This is best shown in 
the obliquely longitudinal section in fig. 64. 
Branching of the Shoot. 
The shoot of Asteroxylon was evidently repeatedly branched, and a number of 
examples must be referred to and several types of branching distinguished. This is 
all that can be done, since the fragmentary nature of the remains is a disadvantage 
in the investigation of this feature of the plant. The type of branching most 
frequently met with is exogenous, and, as shown by the nature of the vascular 
