A Affinities of the Palaeozoic Seeds of the Conostoma Group. 1 1 
width, whilst near the apex it attains a much greater radial thickness 
(114,4 
The vertical measurement appears to vary considerably, reaching a 
maximum of about 323 [i in the body of the seed, though adjacent ele- 
ments cut in the same sense may scarcely reach a quarter of that length. 
’Where the plane of section passes through the curved surfaces of the 
apex and base, the cells of this layer appear to curve respectively for- 
wards and backwards (Text-fig. 4). Since tangential sections of this layer 
are wanting, no direct evidence is available as to their external form ; 
but the appearances described can be satisfactorily explained if we assume 
the exterior wall to have been a hexagon, in which the sides parallel 
to the main axis of the seed were longest, and that these cells were 
arranged in vertical rows (Text-fig. 4, a). Models based on these assump- 
tions, when cut, reproduced with striking fidelity the appearances in the 
various sections. The invariable obliquity of our preparations will account 
for the variations of the palisade cells with regard to their vertical 
dimensions ; the maximum will correspond to cells cut in the direction 
of the long axis of the hexagon, and the minimum to cells cut across the 
corners. 
If a cylinder with longitudinal ribs be cut obliquely it will be found 
that the ribs appear in the sections to slope forwards at the apex and 
backwards near the base ; this is exactly what we find to be the case with 
the palisade cells ; though here the effect is exaggerated both by their 
tapering ends and the fact that their outer surfaces are slightly more exten- 
sive than their inner. Similarly in the oblique transverse section where the 
palisade cells are cut through their tapering ends, these show varied trapezial 
forms, in some places even appearing two-layered. 
The carbonized contents of these cells are usually of a uniform dense 
brown with much lighter or almost colourless walls. In some specimens the 
cells near the apex show a dark body occupying a nearly central position 
and surrounded by a lighter portion resembling a vacuole. In general, they 
resemble the structures seen in the cells of the testa of Stephanospermum , 
only far less well defined (R. 114, PI. I, Fig. 2, t). 
Abutting internally upon the palisade cells were three to four layers of 
fibrous elements constituting the hypoderm ; these were ellipsoidal in trans- 
verse section, the flattening being in the radial direction. So far as can be 
judged from oblique sections, their length varied considerably. In the body 
of the seed this layer reached a thickness of from 57-76 /q and if the single 
transverse section is to be relied upon, was thickest at the two major angles 
and thinnest opposite the four minor. It is chiefly due to increase of this 
tissue that the hard testa attains its greater thickness around the micropyle. 
In contents and nature of the cell-wall the elements of this layer resemble 
those of the palisade. The sections all show the testa cut obliquely, the 
