OF THE FOSSIL PLANTS OF THE COAL-MEASUEES. 
381 
In larger and older specimens I have counted as many as thirteen such laminae in an 
apparently undivided series, though a tangential section of such wedges would probably 
show some thin medullary rays separating the laminae, but which were not observable 
in the transverse section. The following Table gives the mean results of a series of 
measurements of the thicknesses of these compound wedges in a very large stem, 
beginning with those in which each wedge consists of a single linear series of vessels, 
and ascending to those in which I have found as many as thirteen. 
Number of laminte in 
Entire thickness 
Mean thickness of tb 
each wedge. 
of wedge. 
component laminae. 
One roAv of vessels 
. *0023 
•0023 
Two rows ,, . 
. -0041 
•0020 
Three „ 
. -0058 
•0029 
Pour „ 
. *007 
•0017 
Five „ „ 
. -007 
•0014 
Seven „ „ . . 
. -0094 
•0013 
Eleven „ „ . . 
. -015 
•0013 
Thirteen roAvs of vessels . 
. -015 
•0011 
The second column of figures shows that as the wedges increased in size there Avas a 
gradual diminution in the thickness of their component laminae — the result partly of 
their mutual pressure impeding the expansion of the vessels to their normal dimensions, 
and partly of the fact that these larger Avedges contain more than the average of young 
laminae Avhich have been intercalated, and Avhich may not have reached their full 
development. In young stems we find that the dimensions of the wedges are somewhat 
larger than those recorded in the above Table. 
On tracing these wedges in such sections as Plate XXII. fig. 3, from their medullary to 
their peripheral margins, we find that many of them consist of a single lamina of vessels 
throughout their entire course ; but such limited examples rarely, if ever, occur in the 
larger stems. As shown in fig. 3, e, after proceeding outAvards for a little distance, the 
single 1’oav of vessels divides into tAvo smaller ones, which in turn become yet further 
multiplied in the thicker stems. 
These vascular laminae are divided from one another by large medullary rays (f), 
Avhich are very conspicuous, even in the transverse sections, in which they are usually 
composed of several rows of cells Avith thin Avails running in a radial direction, their 
square-ended cells being elongated in the same direction. In tangential sections 
(Plate XXIII. fig. 8) Ave see that the ligneous zone exhibits the appearance of a network,, 
of which the woody laminae (e) represent the threads, and the medullary rays (f) vertically 
elongated lenticular meshes. Two things strike us on regarding such a section, viz. the 
immense number of these rays and their large size. In large stems they frequently have 
a vertical length of a quarter of an inch, and sometimes even more, whilst in such 
specimens T25 is a common size ; their diameter ranges from a thin column of single 
cells up to dense cellular masses (fig. 8,/"'), Avith a diameter of from - 0016 up to *01 , the 
