646 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
The examples figured may now be briefly described. 
The large rhizome in fig. 4 is slightly flattened, but must have been about 6 mm. 
in diameter. Growing within it is another about 3 '5 mm. thick, and in the cortex of 
this are two small specimens under 1 mm. thick. The xylem is evident in all the 
rhizomes, and in the two larger specimens the zone of phloem around it is clearly 
marked. The inner and outer cortex can be distinguished in the largest rhizome, 
but the epidermis had almost completely broken down. 
A specimen of medium size, with the stele divided preparatory to branching, is 
represented in fig. 5, and a similar dichotomy in obliquely longitudinal section in 
fig. 7. The longitudinal section in fig. 6 is of a rhizome of about the same size. 
The strand of xylem is only followed for a short distance, but the section is otherwise 
truly longitudinal. The surface is smooth, the epidermis is intact and well preserved, 
the outer and inner cortex can be distinguished, and the phloem is fairly well 
marked. 
In the small rhizomes (figs. 8, 9, 14, 26-29) the same regions can be distinguished 
as in the larger specimens, but this is not always evident without close examination. 
Thus in fig. 29 the epidermis is wanting, but the regions are fairly well marked, 
and they can also be readily distinguished in fig. 26. In fig. 9 the epidermis is 
wanting, the outer cortex is about two cells deep ; there is a very narrow zone 
of inner cortex and a fairly wide zone of phloem, but the limit between these 
tissues is difficult to distinguish. The junction of inner cortex and phloem is clearer 
in fig. 14. The epidermis is present on the small specimen in fig. 27. The transverse 
■ section on the left in fig. 28 is interesting, since on part of its circumference the 
epidermis is persistent and the surface smooth, while on the other side the 
epidermis has broken down. 
Rhizomes of all sizes evidently branched repeatedly. The most usual method 
seems to have been equal or dichotomous branching, as is shown in figs. 5 and 7. 
In other specimens there are indications that the branching was unequal or 
lateral. It seems always to have been exogenous, and in favourable cases the 
epidermis has been traced from a main axis to a branch. 
The examples of rhizomes described above were all cylindrical. It should be 
mentioned that some other sections have shown a more irregular outline, and 
included several steles. There is nothing to suggest, however, that they were 
anything but cases of irregular subdivision of the rhizome, such as are met with 
in Psilotum. 
The details of structure of the rhizomes can be dealt with briefly, since the 
histology of some of the more important tissues will be more fully considered in 
relation to other parts of the plant. There are, however, some features peculiar 
to these underground parts. 
The epidermal cells are smaller and shallower than those of the outer cortex 
(figs. 6, 27, 28). They are well shown in the longitudinal sections of the outer 
