662 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
In this respect, as well as in the narrow diameter of the tracheides, there is a 
remarkable difference from the xylem of undoubted portions of Asteroxylon. While, 
however, these differences are not to be under-estimated, it should be borne in mind 
that the diameter of the tracheides exhibits such a range in different specimens of 
these axes, and also in different axes of Asteroxylon in the same block of chert, that 
it is not easy to draw a dividing line on account of the size of the tracheides. This 
especially applies to some cylindrical strands of xylem, the tracheides of which, 
though narrow, show the thickening characteristic of Asteroxylon ( cf fig. 104). 
( b ) Sporangia possibly belonging to Asteroxylon. 
The sporangia, which are associated with Asteroxylon and the axes just described 
in the block found by Dr Gordon, differ characteristically from those of the Rhyniacese. 
No confusion with the latter would be possible, and indeed no traces of Rhynia or 
Hornea occur in the peat of this block. The sporangia and spores in question have 
so far only been met with in this one specimen of the chert. 
The spores and the remains of the sporangia occur throughout the, block, but 
more abundantly in certain layers of it. The sporangia are more or less imperfect 
and decayed, and nothing can be said with certainty as to their connection with 
either the undoubted stems of Asteroxylon or with the peculiar axes that make up 
the peat. There is a suggestive association of them with some of the small axes, the 
stele of which is giving off branches (fig. 126), and the impression left by a study of 
the block is, that they were most probably borne on the peculiar axes, and that the 
latter were the sporangiophores of Asteroxylon. 
In many cases only the thickened epidermal layer is preserved. This may be in 
fragments (fig. 124), or may give a fair idea of the general size and shape of the 
sporangium, and show that it dehisced at its wider free end (fig. 123). Here the 
epidermal cells are thickened on their inner and lateral walls, and in the immediate 
neighbourhood of the place of dehiscence they become shallower. The vertical 
section of the epidermal layer thus becomes pointed towards the line of dehiscence, 
as is particularly clearly seen in the two overlapping halves in fig. 125. 
The specimen represented in fig. 126 gives some idea of the shape of the 
sporangium and its rather thick wall. It appears to have been pear-shaped, 
widening out from a thick stalk at the base, and about 1 mm. in length. There are 
some indications that a fairly thick zone of thin-walled tissue had been present 
within the epidermis and formed with this the sporangial wall. 
Other specimens have the numerous spores still enclosed by the sporangial wall 
(fig. 103). The clearest example met with is represented in fig. 123. It is cut 
longitudinally, and shows the pear-shaped outline, with the epidermal layer becoming 
thickened in the upper portion as the place of dehiscence (X) is approached. There are 
remains of thin-walled tissue within the epidermis, and the cavity is filled with spdre§, 
The mature spores are also found free in the peat. They are about 64 ^ in 
