3S6 
PEOJFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
and it further stands related to the equally remarkable fact that no trace of a true leaf 
has yet been met with connected with these stems : and. in but one solitary instance 
have I seen any of the cortical vascular bundles within the area of the prosenchymatous 
zone ; they are limited to the inner bark. To these points, however, I shall shortly 
return. 
Having thus examined the ordinary structure of the Dictyoxijlon Oldhamium , we may 
now turn to some special points connected with its growth. I have already pointed out 
that whilst in some specimens the ligneous zone is thin, consisting of but a very limited 
number of vessels, as in Plate XXII. fig. 4 (which condition is still more obvious in the 
case of the older woody layer, e , of Plate XXIII. fig. 6), I have obtained from Mr. Nield 
one magnificent axis in which the woody cylinder and its contained medulla has been 
at least eighteen inches in circumference. The thickness of the wall of this vascular 
cylinder has been at least inches ; and since the specimen is weathered and water- 
worn*, it may have been of even larger dimensions. In this specimen, assigning to each 
vessel a mean diameter of '004 would give us between six and seven hundred vessels in 
the linear series of each radiating lamina of the wood. Dimensions like these at once 
suggest the existence of some exogenous mode of growth. But fortunately we are not left 
to mere inference on this point. As in the case of the Diploxyloid stems described in my 
last memoir, I have obtained several specimens which unmistakably demonstrate the 
operation of this exogenous development of the Dictyoxylons. Of these, Plate XXII. 
fig. 4 represents one in which there are three distinct rings of vessels; and Plate XXIII. 
fig. 6 is a still more enlarged figure of a second example, drawn with great care, in 
which there are but two concentric growths. In the former example the boundary 
line between the innermost and middle ring is but faint, because the vessels of the 
latter have already expanded to their normal dimensions. The outermost ring 
(Plate XXII. fig. 4, e ') is less completely developed ; hence the vessels have not attained 
to much more than half their size. In Plate XXIII. fig. 6 the distinction between the 
vessels belonging to the inner and older ring ( e ) of the ligneous zone and those of the 
newer one ( e ') is very marked : the latter are of very small size ; not larger, indeed, in 
many examples, than the inner bark-cells (f/) in which they have developed themselves. 
The specimen from which this figure was taken puts beyond the possibility of doubt 
the existence of exogenous growth amongst these plants. 
But there remains another curious evidence of the pseudocambial action of the inner- 
most bark. I have already depicted the usual forms of the cortical vascular bundles in 
Plate XXIII. fig. 7, z, & Plate XXV. fig. 17, z. A similar bundle, copied from the 
specimen shown in Plate XXII. fig. 1, is seen in Plate XXVI. fig. 18. In this example 
the bundle is less sharply defined than in those already described ; and some of the 
vessels in its central portion have the appearance of being intervascular cellular tissue. 
I presume that at least some of this appearance is but the result of imperfect minerali- 
* It was found in a watercourse intersecting the Lower Coal-measures at a locality near Oldham, known as 
Har Culver (Higher Culvert). 
