226 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
invariably find that they proceed from the inner surface of the outer or ligneous cylinder, 
and not from the larger vessels of the medullary one, and it is in the same radiating 
cylinder that we find the medullary rays *. I have already stated my reasons for insisting 
upon the recognition of the medullary character of these rays, and pointed out the 
necessity for considering their primary origin in the nascent structure, prior to any 
material differentiation occurring in its tissues, if we are to arrive at a philosophical 
opinion respecting their nature. All these circumstances combined lead me to the con- 
clusion that in the radiating vascular cylinder we have the representative of the woody 
zone of exogenous plants. This zone is at its minimum of development in such Lepi- 
dodendra as Plate XXIV. fig. 1 & Plate XXVI. fig. 13, whilst it attains to a maximum 
in some of the Diploxylons, the former bearing some such relation to the latter as the 
half-developed woody zone of a Cycad does to that of a hard-wooded Finns or Araucaria. 
This opinion receives further support from the unmistakably exogenous growth of this 
zone. The radiating arrangement of its vessels is suggestive of the conclusion ; but we 
can further see, in many of the stems, clear evidences of interruptions to growth suc- 
ceeded by periods of renewed vital activity. If this reasoning is sound and the conclusion 
arrived at correct, the latter gives us an unmistakable clue to the remaining tissues. The 
thick parenchymatous and prosenchymatous structure investing the woody zone is clearly 
a bark, although not, it is true, divisible into the three layers of epiphlceum, mesopliloeum, 
and endophlceum ; but in the enormous development of elongated prosenchymatous fibres 
or bast-tissues in the inner layer of the epidermis, we have a manifest foreshadowing of 
that prevalence of the same tissue in the bark of living Exogens. M. Broxgniaut has 
already called attention to the close resemblance which the thick cylinder of medullary 
vessels found in his fragment of Sigillaria elegans bore to the ordinary medullary sheath 
of an Exogen, and I cannot resist the conclusion that these are -homologous structures. 
It appears to me that these specimens of fossil Cryptogams explain the development of 
the exogenous medullary sheath, through the gradual separation of the vessels from the 
parenchymatous elements of the pith, until they constitute a distinct ring ; the light 
thus thrown upon their origin further explaining why the ring of spiral vessels never 
recurs in the newer woody layers as they ought to do, if, as has been generally supposed, 
they belong to the inner part of the first formed ligneous zone, rather than to the pith 
which that zone incloses. It appears to me that this reasoning is justified by the facts 
upon which it is based. The principal weak point in it lies in the circumstance that in 
Exogens the spiral vessels supplied to the ribs of the leaves are derived from the meclul- 
* M. Brongniart, in his various writings, distinguishes the Lepidodendra from the Sigillarise by the supposed 
absence from the former of the radiating woody cylinder ; but his knowledge of the structure of the Lepidodendra 
was limited to the one specimen of Lepidoclenclron now become historically famous under the name of L. Har- 
courtii. The series of specimens which I have described demonstrates a gradual transition from the one type 
to the other, with which the French savant was necessarily unacquainted. He concluded that the vascular 
cylinder of L. Harcourtii solely represented the inner vascular cylinder of Diploxylon, which is certainly not 
the case. M. Brongniart had not seen the thin outer ring of small barred vessels occurring in plants of the 
type of L. Harcourtii as seen in my Plate XXV. fig. 14. 
