SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 645 
is particularly interesting. In some blocks of the chert from this region a dark 
sandy band enclosing large stems and rhizomes of Asteroxylon overlies a clear peat 
composed of Hornea and Rhynia major. Rhizomes of Asteroxylon extend down- 
wards in a root-like fashion into the upper portion of the clear peat, sometimes 
penetrating the portions of plants in it. It appears that, in this region at any rate, 
we have Asteroxylon preserved where it grew in the peat. It is clear that the 
rhizomes were the underground . parts of the plant, and behaved physiologically like 
a root-system. This is further shown by the frequent intrusion of these axes into 
other rhizomes and into stems of Asteroxylon itself. 
A general view of a section showing large stems of Asteroxylon embedded in the 
dark sandy matrix overlying clear peat is given in PI. I, fig. 1. In the corresponding 
section given in fig. 2, the sandy band above contains stems of Asteroxylon and a 
rhizome a cut transversely. Two more slender rhizomes, at b, extend downwards 
through the underlying peat, which is composed of more or less decayed Hornea and 
Rhynia major. One of these rhizomes of Asteroxylon has grown through a rhizome 
of Hornea in the peat. The larger rhizome of Asteroxylon at a in the layer above 
contains a similar intrusive rhizome. This behaviour, which is also shown in the 
specimen in fig. 4, recalls a common appearance in rootlets of Stigmaria. 
Intrusive rhizomes of small size have been frequently met with in the stems of 
Asteroxylon , and at first suggested comparison with the endogenous roots enclosed 
in the cortex of some species of Lycopodium. This interpretation was, however, 
negatived by their obvious intrusion in some cases, while in others they were situated 
in the centre of the stele. In fig. 3, two stems of Asteroxylon are shown enclosed in 
a sandy matrix. In the cortex of the large stem above, a small rhizome is cut 
transversely at a. Another rhizome of similar size ( b ) is cut longitudinally as it 
lies free in the matrix. This rhizome passes right across the cortex of the stem, 
, which occupies the lower portion of the figure. 
The largest rhizomes were more than 5 mm. in diameter (fig. 4), and all sizes 
have been met with down to below 1 mm. (fig. 14). A remarkable feature, which 
holds for the rhizomes of all sizes, is the complete absence of absorbent hairs. In 
well-preserved specimens the surface is bounded by a smooth and continuous 
epidermis, but this has frequently broken down and disappeared. 
A general idea of the range in size and structure of the rhizomes will be gathered 
from the examples figured on Pis. I, II, and IV. In the most clearly diflerentiated 
specimens it is possible to distinguish the epidermis (ep.), a narrow zone of outer 
cortex (o.c.), a zone of inner cortex (i.c.), w r hich in the larger rhizomes is relatively 
broad, a clear zone of phloem (ph.), and the central strand of xylem (x.). 
These regions are all shown in a transverse section of a fairly large rhizome 
which was figured in Part I (PI. X, fig. 75), under the mistaken idea that it belonged 
to Rhynia. The general correspondence in construction with the stem of Rhynia is 
brought out by the mistake, which is now corrected, 
