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PEOFESSOE W. C. WILLIAMSON ON THE OEGANIZATION 
lary one. In the minute details of its structure this plant differs in no respect from 
those already described. But we here see that whilst nature has made no attempt to 
reclose the vascular cylinder (<?) and again separate the pith ( a ) from the bark by means 
of the medullary vessels, she has endeavoured to accomplish the same process, though 
not yet effectually, through the instrumentality of the exogenous ligneous zone (d). In 
each of the divisions this exogenous zone overlaps the two free central margins of the 
medullary one, thus gradually filling up the gap between them. I doubt not that even- 
tually such a closure of the vascular ring and isolation of the medullary area would 
become complete. I presume, from the comparative rarity of specimens with these 
open vascular cylinders, that after a growing branch had bifurcated, the buds of the two 
growing twigs have developed their medullary cylinders in the usual way, and that the 
imperfection of the cylindrical ring is confined to the neighbourhood of point of dicho- 
tomization. I have not met with an open ring in a single branch, save when it had 
obviously been ruptured by violence. The specimen (Plate XLIII. fig. 20) is enclosed 
within the usual cylinder of bark (i). 
The last subject brings us to another one on which my views have been criticised by 
some botanists for whose attainments I have the greatest respect, but who have not 
had the advantage of being able to study the large series of specimens which my cabinet 
contains. In both my previous memoirs I expressed my conviction that both in 
Calamites and in the Lepidodendroid plants the peculiarities of their structure could 
only be explained by the recognition of an exogenous mode of growth by which these 
peculiar features had been produced*. My more recent researches have still further 
strengthened these convictions ; so much so, indeed, as not to leave a shadow of a doubt 
on my own mind as to the correctness of my conclusions on this subject. The specimens 
represented in Plate XLIII. fig. 20 and Plate XLII. fig. 11, especially the former of the 
two, afford striking illustrations of this process of growth. The cylinder in the upper half 
of the former figure exhibits no unusual peculiarity ; but the lower one is surrounded 
by a remarkable zone of half-developed vessels ( d '), which is evidently of newer formation 
than the rest of the ligneous zone, and which I can only explain by the assumption that 
it is the product of some equivalent of a cambium-layer. Plate XLIII. fig. 21 represents 
a portion of the exterior of the ligneous zone ( d ), with its radiating lines of vessels (e) 
separated by medullary rays ( f ). Externally to these tissues, we have at e' a new zone 
* My views upon this question having excited so strong an opposition in some quarters, I invited Professor 
Dicksox, of Glasgow, to visit me for the purpose of examining my specimens and giving me his opinion of them. 
He kindly authorizes me to publish the following significant extract from a letter just received from him, dated 
March 17, 1872 : — “ Having examined your sections of stems of Liploxylon showing the outermost woody tubes 
to be of distinctly smaller calibre than the more internal ones, as well as sections of a series of stems of the 
same, from small to large, affording constructive evidence of a progressive increase of the wedge-like woody 
plates, I have no hesitation in expressing my belief in a truly exogenous growth in this plant ; and I consider 
that you are quite justified in applying the terms ‘ medulla,’ ‘ woody zone,’ ‘ medullary rays,’ and ‘ bark ’ to its 
parts, as corresponding more or less perfectly to analogous parts in the Dicotyledonous stem.” — March 19, 
1872. 
