ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 9238 
In some other cases the regular seriation is disturbed by the presence of a more 
or less complete zone of smaller xylem-elements, at first suggesting the idea of an 
annual ring. No such explanation however is admissible, for these small-celled 
regions are of very inconstant occurrence, and, when present, they by no means 
always extend round the entire circumference. Evidently the young wood-cells 
underwent some additional subdivisions in these zones. Beyond them, the regular 
radial series of large elements are resumed. 
Occasionally these slight irregularities co-existed with a very unequal growth of the 
opposite sides of the stem. Extreme cases of this kind have been previously figured.* 
These excessive irregularities are quite exceptional, and were no doubt due to some 
accidental interference with the normal growth. 
The structure of the secondary wood is highly characteristic. The large elements, 
which in transverse section appear nearly square, often with truncated corners, are 
trachee. Their radial walls are marked by numerous small pits (see Plate 83, 
figs. 40, 42, 44, and 444) of somewhat oval outline. When the wall is seen in section 
it appears that these pits were bordered. 
Pits are also sometimes found on the tangential walls, but less constantly. As 
regards the structure of their walls, the secondary trachez are quite similar to those 
of the middle portion of the primary xylem, except that the latter are pitted equally 
on all surfaces (see Plate 83, figs. 40 and 41). 
A more important difference is the fact that the primary elements have pointed 
ends, and are no doubt to be regarded as tracheides, while the secondary tracheze 
appear to form continuous tubes. It is possible that the latter were really vessels, 
but the evidence is insufficient to prove this. We shall see that the supposed remains 
of transverse walls are really of quite a different nature. 
Between the corners of the trachez we find parenchymatous cells, occupying 
the space left free by their truncated angles. (See Plate 75, photographs 20 and 22 ; 
also Plate 83, figs. 43 and 444.) Occasionally one such cell appears in the transverse 
section, in each space; more often there is a little group of them, sometimes six or 
more in number. 
Every here and there the transverse section shows a radially placed cell, or a strand 
of two or three such cells, side by side, passing between the trachez, and uniting 
the parenchymatous groups with one another. 
In a radial section we see that the thin-walled parenchymatous cells at the corners 
of the tracheze, form longitudinal strands of considerable length, which are connected 
at intervals by the radially elongated cells. (See Plate 83, figs. 44 and 44a.) 
In a tangential section of the wood, only the lenticular cross-sections of the 
horizontal parenchymatous cells are seen, for none of them lie in a tangential plane.t 
It was these radial parenchymatous cells which M. Renavtt at one time regarded 
* Wi.uiamsoy, loc. cit., Part V., Plate 2, figs. 11 and 12. 
+ See Witiramson, loc, cit., Part V., Plate 2, fig. 13. 
