392 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
to the external surface of each wedge. Examples of all these methods of growth abound. 
If not antagonized, the continuance of this process would have given the peripheral por- 
tions of the wedges very different dimensions to those described in the preceding pages. 
I have already pointed out that we rarely find a dozen laminae in any one wedge. The 
constant intercalation of new secondary medullary rays which took place here, as in living 
exogens, produced the result described. We thus see that in many of its minute details, 
as well as in its more general aspects, the growth of these stems has been a truly 
exogenous one. 
The bark of one specimen, but which stands alone amongst the number of those that 
I have examined, exhibits conditions requiring further elucidation : I am as yet unable 
fully to understand its significance, but it is suggestive of additional facts belonging to 
the history of Dictyoxylon that have not yet been worked out. In this specimen, part 
of which is represented by Plate XXVI. fig. 25, w T e have the exterior of the ligneous zone, 
at d. Some very slight traces of the parenchymatous layer of the bark are to be seen 
investing the woody zone. On the other hand, the prosenchymatous layer (k) is very thick : 
this thickness is due to the appearance of a large double vascular bundle, surrounded 
by a considerable mass of parenchyma, which is lodged in the substance of the prosen- 
chymatous layer. Two of the vertical prosenchymatous laminae, seen at 7t J , Jc', have obvi- 
ously been pushed aside to make room for the above intrusive structure. I have already 
mentioned the fact that I have never seen any of the cortical bundles located in the pro- 
senchymatous zone of the bark, save in one solitary instance, the latter reference being 
to the example now under consideration. The double vascular mass (z) bears so striking 
a resemblance to these bundles when but partially divided, as to leave little doubt that 
it is really one of them. At the first glance we might suppose that it had been acci- 
dentally displaced from its normal position and forced, by pressure, into the outer bark ; 
but such is certainly not the case. The parenchyma with which it is surrounded is 
identical with that separating all the prosenchymatous laminae, leaving little reason to 
question that one of the parenchymatous areolae of the outer bark has been specially 
enlarged to admit of the outward transmission of the vascular bundle and its thick paren- 
chymatous investment, the vertical prosenchymatous layers (//, /;) having been thrust 
apart to make room for this enlargement and transmission. 
These conditions suggest the possibility that others of the cortical bundles may finally 
leave the woody zone and pass outwards to supply foliar appendages with vascular tissue. 
The large size of these bundles indicates (supposing the above suggestion to be correct) 
that the missing appendages have been of corresponding dimensions. They suggest 
fronds rather than leaves. It is impossible to reason upon such limited data with any 
confidence ; but I shall recall attention to the facts just described when I proceed, on some 
future occasion, to examine the stems or petioles to which I have proposed to assign the 
provisional name of Edraxylon *. 
* Proceedings of the Royal Society, vol. xx. p. 438 (1872). 
