ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 881 
bably no definite interval between the primary and secondary tissue-formation, hence 
it is convenient to take the interfascicular divisions as marking the commencement of 
secondary growth. In fig. 2 then, we have a stage immediately before, and in fig. 1, 
a stage immediately after the beginning of cambial increase. We can scarcely expect 
to fix the starting-point more exactly than this. 
In describing the secondary tissues we will begin with the fascicular wood. The 
elements are arranged with remarkable regularity in radial series. Passing from within 
outwards, the number of the series gradually increases, by the occasional duplication 
of a row, but the regularity of the tissue is always maintained, 
In good transverse sections we can easily see that the wood consists of two kinds 
of elements; the majority of the radial series are composed of relatively large elements, 
with rather thick walls, while other rows, between the former, consist of narrower 
cells, with thinner walls (see Plate 72, photograph 3; Plate 77, fig. 3; Plate 78, 
fig. 12, &c.). The comparison of tangential and radial sections show that the former 
elements are tracheze, the latter constitute small secondary medullary rays (see 
Plate 72, photograph 6, also WinLIAMson, “Organization,” Part I, Plate 23, fig. 5 ; 
Part IX., Plate 20, fig. 16). As we advance from within, outwards, we find that the 
number of these secondary rays increases, new ones making their appearance suc- 
cessively, just as happens in the secondary wood of a Gymnosperm or a Dicotyledon. 
Most often the secondary rays are one cell only in breadth, frequently they are two 
cells broad, sometimes more. The radial diameter of their cells is equal to, or some- 
what greater than that of the trachee; their tangential diameter is much less. 
The height of the ray-cells is generally their greatest dimension, so that they belong 
to the “upright” type of DE Bary.* In other cases, however, the ray-cells are 
approximately square, as seen in radial section, and the two kinds of elements may 
occur in the same ray. Uniseriate rays (only one cell in height) are very common ; 
their form, as appearing in a tangential section, is lenticular. 
The proportion of tracheze to secondary rays varies in different specimens ; the 
tracheal series, however, are always the more numerous. 
As regards the nature of the trachez, the most important question which we have 
to decide, is whether they are true vessels or tracheides. These elements are of very 
considerable length, and have very oblique terminal walls, inclined to the radial plane 
at an acute angle (see figs. 16 and 17, on Plate 79). It is not an easy matter to 
measure their length, as the whole trachea is not often included in the plane of section, 
and it is only in the best-preserved specimens that it is even possible to determine 
whether this is the case or not. We have, however, succeeded in making some 
measurements. In a tangential preparation (C.N. 20 B, from part of which fig. 8 on 
Plate 78 was drawn), five tracheze were measured ; their length varied from 1°7 to 
2°4 millims., the average being about 2 millims. This, however, is much exceeded in 
other cases. In another tangential section, perhaps the most perfect in the collection 
* Toe, cit., p. 486. 
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