312 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
medullary cylinder appears to correspond with the pith-crown (Markkrone) or pith-sheath 
(Markscheide) of the Germans ; only it lacks, in the fossil forms, all the multiplied 
wood-cells of various kinds which enter into the composition of the Xylem portion of 
that structure, whilst the Phloem layer has no true representative either at this or any 
subsequent stage of growth. 
With the exogenous peripheral extension of the wood some new differences present 
themselves. In the case of the Dicotyledons and Gymnosperms, the fibro-vascular 
bundles of the medullary sheath, or “ pith-crown,” consist of elongated, annular, spiral, 
and reticular vessels, mingled with long wood-fibres ; whilst in the new layers of secondary 
wood no spiral or annular vessels appear, their places being taken by what Sachs terms 
“ short-membered, wider, pitted or dotted vessels”*. In the fossil Lycopods, as we have 
seen, the first exogenous zone is developed immediately around the vascular medullary 
cylinder, just as the first layer of secondary wood is developed immediately around the 
medullary sheath in the Dicotyledons and Gymnosperms. But instead of a change 
occurring in the nature of the vessels in such new layers of these Lycopods, corre- 
sponding with that just referred to in the living Exogens, the vessels of the new zone 
of the former are mostly identical in character, except in their smaller size, with those 
of the medullary vascular cylinder. If the former are barred, so are the latter ; if the 
former are reticulated, so are the latter. But with this exception, the further development 
of these new zones proceeds so as to produce results substantially representing those 
seen in living Exogens. Thus a ring of detached vascular bundles first surrounds the 
vascular medullary sheath with definite vertical layers of mural cells between them, 
constituting the beginning of as many medullary rays. New bundles are added to the 
exteriors of the preexisting ones, as well as new cells to the peripheral margins of the 
medullary rays. As this intercalation of additional radiating vascular laminae increases 
the tangential diameter of these bundles, new, and yet more peripheral, medullary rays 
become intercalated, as in living Exogens ; so that, though these exogenous zones have' 
attained, in many of the fossil Lycopods, to very large dimensions, no material increase 
takes place in the diameter of the individual woody wedges as they progress from within 
outwards. I have also shown in the preceding pages that these exogenous layers neither 
contribute to nor receive from the leaves any portion of their vascular elements ; whilst, 
as shown in the case of the Stigmaria , they do furnish the vascular bundles going to the 
rootlets, and consequently act as the channels through which the crude sap has ascended 
from the roots to the upper portions of the tree. It appears to me that we have here an 
analogy of the utmost importance in relation to the problems under discussion. We 
seem to have here an identity of function which overrides all secondary differences of 
origin in its bearing upon the nature and homologies of these several structures, and 
which, when superadded to the structural resemblances existing between the exogenous 
ligneous zone of a Diploxyloid Lycopod and that of a recent Dicotyledonous tree, justifies 
my hypothesis as to the relations subsisting between the two in no slight degree, viz. that 
* Lehrbuch, p. 540. 
