652 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
Two small twigs magnified 20 diameters are represented in fig. 54, and show 
the proportional size of the solid triangular or quadrangular strand of xylem and 
the stem itself better than those in fig. 48. The soft tissues are, however, very 
poorly preserved. The bays in the angled xylem are here shallow, and there is some 
reason to think that in some ultimate branches the xylem formed a cylindrical 
strand, though no well-preserved specimens of this have been seen (cf the 
specimens in fig. 47, on the right). 
The stem in fig. 49 is a good example of one in which the inner cortex is 
composed of uniform tissue with large intercellular spaces, and is not differentiated 
into zones. The narrow outer cortex is distinct from this. The obliquely cut stele 
is giving off large traces, one of which is probably for a branch and the other a 
leaf-trace. 
The arrangement of the tissues in the stele has been briefly described above, and 
was evident in the figures of complete transverse sections of stems on PL VI. A 
number of steles are represented somewhat more highly magnified on PI. VII, and 
show the details better. Thus fig. 50 is the stele of a small stem, with the xylem 
three-rayed, and the ends of the rays gradually widening. The phloem is fairly well 
preserved in the bays. Fig. 53 is a typical large stele as usually preserved. The 
ends of the four rays of the xylem are greatly expanded, and leaf-traces are 
departing from them. The phloem in the bays is only partially decayed, but that 
around the xylem has collapsed. This stele may be compared with the specimen 
in fig. 72, in which the phloem is exceptionally well preserved. The stele in fig. 57 
shows only the xylem well, but this is of interest on account of the more slender 
three-rayed central portion which connects the widened and divided ends of the 
rays from which the small leaves-traces depart. In the stele in fig. 52 the central 
connecting portion of the xylem is imperfectly developed, and the numerous widened 
ends are partially isolated. In fig. 51 the well-preserved xylem of the stele is 
separated into distinct masses by the phloem extending across. 
Before passing to consider histological details, mention must be made of some 
sections of the shoot in which the imperfect differentiation of the tissues suggests 
nearness to the apical region. The specimen represented in fig. 58 can only be 
interpreted as of this nature. It is an accurately transverse section of a twig, the 
outline of which is irregular owing to the projecting leaves. The tissues of these 
and of the epidermis and outer cortex, with which they are continuous, appear almost 
mature and of full thickness. The inner cortex, on the other hand, is narrow and 
immature, though it shows the distinction of zones. The stele is represented by the 
central clear area, with no indication of developed xylem, a tissue which in Astero- 
xylon is very persistent. The stele thus appears completely undifferentiated, and, 
unlike the cortex, does not show the cells clearly. 
Fig. 59 is of a specimen of somewhat different character, and appears to belong 
to a leafy shoot of fair size. It bears leaf-bases, and the outer and inner cortex have 
