46 
PEOEESSOE W. C. WILLIAMSON ON THE OEGANIZATION 
the irregularity of these growths, owing to some of the layers being crescentic ones, 
instead of forming perfect circles, became greater. The hark (A) in this example also 
consists of the middle portion. Both the last figures show very clearly the radiating 
arrangement which the long prismatic cells of this tissue exhibit when divided trans- 
versely. 
In Plate II. fig. 11 we not only advance yet further, but we now discover that a 
change has been introduced into the method of growth. The specimen, for which I 
am indebted to Mr. Whittaker, of Oldham, has been somewhat crushed, but this in no 
way interferes with the easy interpretation of it. Its central portion has developed in 
the manner described in the case of the preceding examples, the triple arrangement of 
large vessels alternating with corresponding radiating groups of smaller ones. But when, 
by successive growths, the vascular axis attained the condition of a perfect circle, an 
altogether new mode of procedure has manifested itself. The preexisting axis has 
become the starting-point for a new series of layers, in which all the vessels are abruptly 
reduced to a very small and nearly equal diameter. Hence the older and the newer 
growths are separated from one another by a very distinct circular line of demarcation. 
Prom the new starting-point the vessels have been disposed as before, but they have 
increased slowly in size as each succeeding layer invested its predecessors. The entire 
stem in this instance has only the same diameter as that of Plate II. fig. 9 ; but I have no 
reason for supposing, on this ground, that these two belong to distinct plants. I have else- 
where obtained clear evidence that stems having the same diameter may nevertheless have 
attained to very different stages of growth. Plate II. fig. 12 is a second section seen in the 
same slide as the last, and is evidently part of another stem or branch of the same plant. 
The remarkable difference in the sizes of the larger vessels of the central growths seen 
in the three radiating clusters of them, as compared with the very small vessels com- 
posing the later and more external series, is here very striking. At first sight it is diffi- 
cult to avoid the conviction that the difference is due to some increase in the size of the 
former, as compared with the corresponding ones of the previous examples ; but such 
is not the case. The largest of those in Plate II. fig. 12 is not more than '005, which we 
have already found to be a usual size. The difference occasioning the striking contrast 
has resulted from the very small dimensions of those composing what may be designated 
the secondary growths. These last two examples exhibit the highest degree of develop- 
ment which I have met with in any stem found in the Lancashire district *. 
I succeeded in obtaining a very accurate tangential section of the exogenous portion 
of Plate II. fig. 9, with the object of ascertaining the exact distribution of the 
medullary rays separating the several radiating laminae of vascular tissue. Plate II. 
fig. 13 represents a portion of this section. The medullary rays are numerous, but of 
* Since the above two figures were drawn, I have met with a very perfect example of the same stage of 
growth, in which the tissues are wholly undisturbed by external pressure. It fully confirms my conclusion 
that the conditions just described are not due to any such disturbance, excepting so far as the accidental dis- 
placement of some of the tissues is concerned, hut are characteristic of this stage of the growth of the stem. 
