20 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
young lateral twig of Bignonia the ligneous axis is nearly cylindrical, when it must be 
invested by a complete ring of cambium ; but this ring clearly becomes broken up, as 
growth advances, into four detached segments, corresponding to the peripheral extremities 
of the four large crucially disposed woody wedges seen in more matured stems. Hence 
in this case the cambial ring, primarily continuous, became an interrupted one. It 
subsequently again approximated to a continuous one by the development of interfas- 
cicular cambium. But whilst on these points there seems to be some analogy between 
the development of a Bignoniaceous twig and that of my fossil plant, there yet remains 
an additional distinctive feature. In the former a detached patch of cambium occupies 
the innermost extremity of each of the four centripetal prolongations of the inner bark 
which separate the woody wedges, so that these cortical radii are steadily encroached 
upon by four corresponding radii of wood, which are slowly growing outwards, following, 
but at a very great distance, the extensions of the four arms of the cross. Nothing like 
this occurs in my Carboniferous plant. In the latter the six centripetal extensions of the 
inner bark never become thus converted into a portion of the vascular cylinder. 
Figures 37 & 38 represent two transverse sections of the vascular axes of stems of 
small size. The larger of these has a diameter of ’032, and the smaller one of ‘017. The 
larger of these two sections is therefore only about two thirds the size of the correspond- 
ing vascular axes of the young branch in fig. 34, the diameter of which is about ’047, 
whilst the smaller one is but little more than one third of the diameter of the same 
branch. The vessels in figs. 37 & 38, like those of the branch referred to, have a maxi- 
mum diameter of '0025. I conclude that these two figures represent the vascular 
centres of two small branches deprived of their cortical layer. Their chief interest lies 
in the circumstance that fig. 37 exhibits but five primary vascular wedges, and fig. 38 
but four ; whereas every example which I have yet seen, where the stem has attained 
to what appears to be its normal dimensions, these primary wedges are six in number, as 
already described. The section of a nodule which contains the two stems (figs, 37 & 38) 
also contains a third one in their immediate vicinity, and which in all probability 
belonged to the same plant as the other two : its vascular axis has a diameter of rather 
more than T. It also has but five wedges ; but one of them is so much broader than 
the rest as to make it exceedingly probable that it would eventually subdivide into two, 
and thus complete what appears to be the complementary number of six, characteristic 
of all matured stems. 
The above description makes it obvious that in a very young stage of the development 
of the exogenous, vascular portion of a stem or branch, the differentiation of the vascular 
laminae into the six distinct groups, which I have termed primary wedges, is very imper- 
fect, the large radial masses of cellular tissue {h) which ultimately separate them being 
then little more than ordinary medullary rays. If this is true, the mural parenchyma 
composing them must retain its genetic properties as a meristem tissue, since they cer- 
tainly become much broader, containing an increased number of cells, as they grow 
older. 
