SHOWING STRUCTURE, FROM THE RIIYNIE CHERT BED, ABERDEENSHIRE. 883 
of the tissue are on the whole cut transversely in this section, which may be pre- 
sumed to be transverse to the long axis of the portion of the plant from which the 
fragment comes. A number of approximately circular areas from which the wide 
tubes are absent are shown; for these the term “medullary spots”* may be 
employed. A portion of the section is magnified 100 diameters in fig. 110, and two 
other portions are magnified 250 diameters in figs. Ill and 112. The last-named 
figure shows a portion of one.of the medullary spots surrounded by the tissue formed 
of large tubes. 
The relatively large tubes show considerable range in size, but those of different 
diameter are mixed together and not arranged in concentric zones as in Nema- 
tophyton Logani. The largest have a diameter of 52 m, while the smaller ones 
measure only 20 m. They are frequently contiguous, often forming rows or small 
groups, while at other places they are distinctly separated from one another. The 
intervening spaces, like the medullary spots, were occupied by the system of fine 
tubes or hyphae to be described below. 
The walls of the larger tubes are usually very thick and often appear thicker at 
one side than at the other. Thus in a tube 34 m in diameter the thickness of the 
wall ranged from 4 m to 7 m. Another tube 48 m in diameter had the wall 14 m thick. 
Other tubes had thinner walls, the wall of one with a diameter of 20 m being only 
2 m thick. The tubes thus vary from 20-52 m in diameter, with a wall-thickness 
varying from 2 m to 14 m or more. The apparent thickness of the wall may have 
been affected by the state of preservation of the tube. 
The tubes so far described are cut transversely, but others lie obliquely and are 
therefore seen from the side or in longitudinal section as they lie between the other 
tubes. These obliquely running tubes are sometimes relatively narrow, down to 
12 m, but belong to the system of relatively wide tubes. Some of them can be 
traced into the medullary spots. 
Where the relatively large tubes are not in close contact, and especially in the 
areas of the medullary spots, there is evidence of the presence of a second system 
of delicate tubes of about 10 m in diameter. In most places between the wide tubes 
and in most of the medullary spots the tissue formed by the narrow tubes has lost its 
structure and is either amorphous or replaced by a pseudo-cellular structure. In 
one or two of the medullary spots, however, the delicate tubes could be more or less 
clearly followed, and we have no doubt that they were originally present in the 
corresponding situations from which they have decayed. 
It will be seen that, small as this specimen is, it shows all the types of tissue 
described for other species of Nematophyton. 
It is not necessary or desirable to press the description of the tissues of this 
small fragment of Nematophyton further. It is sufficient to be able to recognise 
that it agrees with other specimens of this genus in the absence of true tissues such 
* Penhallow, Ann. Bot.,v ol. x, p. 144, 1896. 
