ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 883 
overlap, so that each trachea abuts on portions of two others in the next row, we 
find the shorter pits. 
Where tangential pits are present, 2.e., on the walls of the more internal trachee, 
they are usually scalariform. Sometimes a few oval pits are seen on the tangential 
walls in the transitional region, before they disappear altogether. 
The very oblique terminal walls of the trachee have similar pits to those of the 
radial walls, from which indeed they are in no way marked off. 
In tangential sections we frequently find the pits well shown in sectional view.* 
In all cases they are evidently bordered, the thickened ridges distinctly over-arching 
the delicate closing membrane, which is, of course, only preserved in the most 
favourable cases. Between two tracheze the pits are bordered on both sides ; between 
a trachea and a cell belonging to a medullary ray, whether principal or secondary, we 
find one-sided bordered pits, the border being on the side towards the trachea. (See 
Plate 78, fig. 9, a and B.) 
From the general form of the tracheal elements, and especially from their tapering 
ends, the impression is strongly conveyed that they are tracheides and not vessels. 
Their oblique terminal walls have the same pitted structure as the lateral walls, and 
there is no evidence that the pits were perforated in either case, though, of course, it 
is impossible to prove that this never happened. Direct evidence of the development 
cannot be expected from a fossil, but when we come to consider the interfascicular 
wood we shall find some facts which speak strongly for the origin of each trachea from 
a single cell. There is no reason to doubt that the same mode of origin held good 
for the fascicular trachee. 
In exceptional cases, however, we have found occasional traces of transverse walls 
in the tracheze. Though such traces are sometimes doubtful, they are not always so. 
In a very few instancest the transverse wall is unmistakeable, and indeed seems to 
have formed a permanent septum. It must be understood, however, that such 
indications of septa are extremely rare, only occurring in certain preparations, and in 
a few tracheze in each case. In those tracheze which show the transverse walls, their 
position is quite irregular. We by no means believe that this casual appearance of 
an occasional septum points to the origin of the tracheze by cell-fusion. It is doubtful 
whether the septa, when present, were ever absorbed at all. Two explanations are 
possible ; either the young tracheide, while still a living cell, occasionally underwent 
septation, as is sometimes the case with sclerenchymatous fibres,{ or else these 
transverse walls are not normal, but mark the limit between the cells of a thylosis. 
There is no evidence for the existence of any xylem-parenchyma, apart from the 
medullary rays, in the secondary wood. Radial sections are decisive on this point, 
for all the parenchymatous cells manifestly form part of radial plates of muriform 
* They are shown in great perfection in O.N. 20a and zB, 137* and 138*, 1554 and 1937. 
+ Such an instance occurs in C.N. 138*. 
{ See pr Bary, loc. cit., p. 134, 
