S topes.- — The Missing Link in Osmundites . 
57 
Detailed Description of Specimen. 
The main axis is a solid mass of wood. Photographs of the two good 
sections of this stele are shown on PL II, Photos 2 and 3 ; in Photos 4 and 5 
are shown lower magnifications of the whole sections in which they lie. These 
make clear the relation and relative sizes of the leaf-bases surrounding it and 
of the main axis. The total diameter of the xylem of the main stele 
is roughly 1 mm., and the whole is irregularly circular with a slight tendency 
to a stellate shape. (This is of historic interest in comparison with the 
palaeozoic forms, such as Zalesskya , which Kidston and Gwynne-Vaughan 
link on to the group ; see their memoirs quoted above.) 
The whole of the central axial tissue appears to me to be tracheal, 
but about this I cannot be absolutely sure, owing to the absence of longitu- 
dinal sections. In the actual slides the solid nature of this central mass is 
very much more evident than in the photographs, particularly in the slide 
(V. 12641 h) from which Photo 3 is photographed. In this, when the high 
Diagram of leaf-base of Osmundites Kidstoni , sp. nov., showing the position of themeristele and 
sclerotic tissues ; the latter are dotted, x 15. 
power is used with a much reduced orifice in the iris diaphragm, it reveals 
cell structure right across the space which, in the photograph, looks like a 
gap. The preservation of the cell-walls here is not optically sharply defined 
from the mineral matrix, while the cell-walls which show up clearly in the 
photographs have a strongly-marked contrast with the matrix, due to their 
black colour. So far as I can judge, this appears to be due to the chances 
of petrifaction, and not to any inherent differences in the natures of the cells. 
This interpretation is confirmed by the presence of a single cell or two which 
has one half of its walls black and strong, and the other half faint and trans- 
lucent, and therefore, in the photograph, invisible. 
Whether all the central cells were tracheides, or whether dispersed among 
them were some parenchyma cells, absence of longitudinal sections of this 
axis makes it impossible to say with certainty, but the appearance in trans- 
verse section shows a certain amount of the pitting, and there is no conclusive 
evidence against the view that the mass consisted almost entirely of 
tracheides. 
The protoxylems are not clearly evident, but seem to lie somewhat 
