ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 939 
Quite at the end of the sporangium, however, we often find a group of specially narrow 
cells, suggesting that here dehiscence took place. Measurements give the following 
average results :— 
Large epidermal cells of pedicel . . . +12 millim. radial diameter. 
Largest cells of sporangial wall . . . ‘ly rr ie a 
Ordinary cells of wall. . . . 2. . . 073, e 
Narrowest cells of wall . . . . . ) . 036 
What has just been said answers another of M. ZetiuEr’s enquires (loc. cit. p. 23), 
namely, whether the largest cells belong exclusively to the pedicel, or whether some 
also belong to the sporangium itself. Undoubtedly the latter is the case. 
A third point suggested by M. Zertuer for investigation relates to the thickening 
of the membranes of the large cells in question. He compares them to the annulus 
in Ferns, and enquires whether, like the cells of that organ, they are more thickened 
on their inner than on their outer surface. We have carefully examined the cells 
both of the pedicel, and of the sporangial wall, from this point of view. They are 
quite similar to each other, and their membranes show a distinct thickening in the 
form of bars, running along their lateral walls in the radial direction. In some cases 
the thickened ridges extend also over the inner and outer cell-walls, but we could 
detect no difference between them as to thickness. 
The Spores. 
The spores of Sphenophyllum Daawsoni are already well known; their cell-wall 
possesses a highly characteristic ornamentation, consisting of prominent spines, which 
are connected together on the external surface of the exosporium, by a reticulum of 
elevated ridges.* This structure is quite constant ; in one strobilus only did we find 
any deviation from it. In this specimen only two of the sporangia shown in the 
sectiont contain typical spores, which are here about ‘1 millim. in diameter. The 
other sporangia contain thin-walled spores, without spines, and of somewhat smaller 
size, though not smaller than the typical spores of some other specimens. It is 
probable that the thin-walled spores in this strobilus were immature. 
There is nothing whatever, in any of our specimens, to suggest that the fructifica- 
tion was heterosporous. As about a dozen distinct specimens have now been examined, 
it is highly probable that if two kinds of spores were present, both must have been 
observed. The variations in the size of the spores are inconsiderable, ‘076 millim. 
to ‘104 millim. being the extreme range ; these extremes are connected by spores of 
intermediate dimensions. 
* See WILLIAMSON, loc. cit., Part XVIIL., Plate 25, figs. 17 and 18. See also our own figures 57 and 58, 
on Plate 85. 
+ C.N. 1049 C, figured by Wiutiamsoy, loc. cit., Part XVIII., Plate 28, fig. 6. 
