Or THE FOSSIL PLANTS OF THE COAL-MEASUEES. 
481 
seems an appropriate one to apply to them. In some parts of his Monograph Mr. Binney 
describes them as having “ their walls perforated with oval openings.” This is certainly 
not the case, the supposed openings being merely thin places in the tubes where the 
original cellulose wall has remained unthickened by secondary internal deposits of lignine. 
These vessels approach very closely to the true annular vessel ; but I have never yet seen 
an example in which individual rings could be traced apart from their neighbours, as is so 
constantly observable amongst recent annular tissues. As these peculiar vessels are very 
common amongst the plants of the Coal-measures, occurring in the Sigillarise and other 
genera as well as in Calamites, it seems desirable that they should be distinguished by 
some appropriate name, and the term “barred” appears to answer the purpose. The 
“ reticulate ” vessels so common amongst Calamites are but modifications of the same 
structure, occurring in the same stems with the barred varieties, and not unfrequently 
the two appear to be convertible. In several instances I have noticed that the vessels 
at the inner extremity of the wedge were barred, whilst those constituting its peripheral 
portion were reticulated. In many specimens, though the boundaries of the vessels are 
well defined, all trace of their internal organization has disappeared. Such examples 
appear as represented in the vessels (g) of fig. 3 ; but I believe that this is an abnormal 
condition due to imperfect fossilization, since we can constantly trace the transition from 
the smooth to the barred and reticulated forms. The vessels at the medullary or inner 
extremity of each radiating lamina are, as Mr. Binney has correctly pointed out, fre- 
quently smaller than the peripheral ones, though the difference is not always very marked. 
The largest vessels in each specimen range from •003 to -006 of an inch; the smaller 
ones are often less than half these diameters. Their number in a linear series varies 
with the age of the plant and with the part from which the transverse section is taken. 
In very young plants they are but few, whilst in one specimen I counted 354 in one row ; 
as will be seen shortly, they are more numerous at the nodes than at the internodes. 
The number of the woody wedges, as well as the distances between the canals whence 
they spring, varies with age and other circumstances. I have discovered no evidence 
leading me to believe that the number of the wedges was increased after a young 
shoot, however minute, was once organized. In some instances I have counted as few 
as fifteen, and Mr. Binney has figured an example with but nine. On the other hand, 
I have counted as many as eighty in a transverse section of a stem little more than half 
an inch in diameter ; in one old arenaceous cast I found that the outer surface indicated 
ninety of these wedges. Indications of wedges intercalated after the first growth had 
begun have only been met with in one example, in which a solitary wedge first appears 
at a point a little external to the concentric line formed by the inner angles of the rest 
of the series, and it was devoid of the usual accompanying longitudinal canal. This 
absence of regularly intercalated wedges has some physiological significance in relation 
to the age and growth of these stems. The distance intervening between contiguous longi- 
tudinal canals practically marks that existing between the centres of contiguous wedges. 
This often varies somewhat in the same section, though the variation is limited. In the 
