OF THE FOSSIL PLANTS OF THE COAL-MEASTJBES. 
293 
in process of development ; it consists of numerous masses of small vessels arranged, in 
the transverse section, in a radiating direction, but of which the lines have not yet 
assumed the orderly disposition that characterizes them when fully developed. Between 
these vascular laminae are cellular masses (/''), the positions and structure of which 
obviously show that they are destined to become prolongations of the medullary 
rays (f). Plate XLIII. fig. 22 represents part of a tangential section of the new tissue 
(fig. 21, which is very instructive. The right-hand portion of the section dips 
more deeply into the specimen than that to the left ; the latter consequently exhibits the 
more peripheral aspect of the structure. In the former the vessels are becoming closely 
arranged, and the medullary rays (f), though still much more enlarged and containing 
more cells than characterize the matured rays of the woody zone, are comparatively 
circumscribed ; but in the more peripheral part the vessels (e") are more widely sepa- 
rated, meandering through large cellular masses (f"), which are scarcely, if at all, distin- 
guishable from the contiguous parenchymatous bark-cells. These young vessels have a 
diameter of from ’0025 to ’0012, whilst the transverse bars on their walls are from ‘0003 
to ‘0002 apart. In the matured vessels we have a diameter of from ‘005 to '0024, whilst 
the bars are from -0008 to ‘00035 apart. The comparison of these figures demonstrates 
that the young vessels under consideration are but half-developed in either direction ; both 
in their diameter and in the longitudinal separation of their bars of lignine they must have 
attained to double their present dimensions before they corresponded with those of the 
matured ligneous cylinder which they invest. At this early stage of their growth the 
walls of these vessels exhibit a crenulated outline, the indentations being caused by the 
pressure of the contiguous cells upon the half-plastic tissues. This feature disappears 
as the vessels swell to their full dimensions and are brought into mutual contact by the 
absorption of the cells which temporarily separate them ; but it is permanently main- 
tained where the vessels are in contact with the medullary rays. I have not been able 
to identify any of the cellular structures that surround them with true cambium-cells : 
though exceedingly delicate they have the aspect of formed tissues ; but there is not 
the slightest room for doubting that both cells and vessels are younger than those of 
the ligneous zone which they enclose, or that they are the products of an exogenous 
growth in which the Xylem of the German botanists is represented, whilst the Phloem is 
absent*. 
I have called attention to the break in the continuity of the medullary cylinders of 
Plate XLIII. fig. 20, through which a direct communication is established between the 
cells of the medulla and those of the bark. The equivalent of the cambium has bent 
round the two inner horns of the crescent-shaped medullary cylinder and formed the 
I may observe here that since my last memoir was written I have obtained specimens of Stigmaria which 
exhibit conditions very similar to those of the example of Diploocylon just described, but in which the growth of 
the new vessels is rather more advanced. I have noticed that in Stigmaria the additional growths are rarely 
made in complete circles, but rather in layers having crescentic transverse sections ; I have found the same 
conditions in some other plants from the Coal-measures yet to be described. 
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