314 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
the publication of that memoir, and the identity of which with those represented by 
fig. 52 is demonstrated by their possessing in situ the peculiar rootlets so characteristic of 
Stigmaria. I have not obtained similarly conspicuous proofs of this intermittent cell- 
action in the innermost bark ; but the periodic additions made to the exterior of the 
exogenous vascular zone, as illustrated oy fig. 21 of the present memoir, demonstrate 
that similar alternations of activity and rest must have occurred in this region. Hence 
we appear to have in these Cryptogamic Lepidodendroid stems two concentric vertical 
zones in which these alternations occurred, — one in the same region as is occupied in 
Dicotyledons and Gymnosperms by the true cambium-layer, and the other in the same 
plane as that which contributes to the growth of the cork-layer of the bark in the same 
plants*; and whilst fully recognizing the differences between the details of the physio- 
logical phenomena in the two classes of instances thus compared, I cannot believe that 
the coincidences referred to are wholly accidental. Be that, however, as it may, we are 
brought to the conclusion that though the accessory phenomena attending the exogenous 
growth of the stems of these fossil Cryptogams differ from those seen amongst the recent 
Dicotyledons, that process of growth practically leads to similar results in both cases, so 
far as the lateral expansion of the woody zone is concerned. The most striking difference 
between them lies in the entire absence of ligneous prosenchyma or wood-cells from the 
exogenous zone of the fossil types ; but this is not more remarkable than the equally 
complete absence of the other, or vascular, element from the corresponding zones of a 
coniferous stem. In a letter which I recently received from Professor Sachs, referring 
to this subject, he says: — “The main point seems to me to be whether or not, in the case 
of Cryptogams, subsequent growth in thickness (Dickenwachsthum) occurs. Whether 
this takes place by means of cambium or merely by means of meristem, is manifestly a 
question of secondary importance.” That such a growth does occur is now put beyond 
all possibility of doubt. 
But the difficulties which surround these efforts to ascertain the homologies subsisting 
between the Carboniferous Lycopocls and living plants are not confined to the exogenous 
zone. A somewhat similar difficulty attends the attempt to establish true homologies 
between the vascular medullary cylinder of the plants described in this and the preceding 
memoir and the central fibro-vascular bundles of the living Lycopods. That these two 
structures are homologous I have no doubt, nor, so far as I am aware, has any other 
observer. Professor Sachs agrees with this conclusion. He writes, “ I consider that 
your medullary axis of fibro-vascular bundles consists of several such fibro-vascular 
bodies as I have depicted in fig. 310 in the second edition of my ‘ Textbook,’ ” — which 
figures represent sections of the stems of living Lycopods. But more than one difficulty 
presents itself when we try to work out the details of this relationship. 
Our fossil Lepidodendra do not exhibit any thing which exactly corresponds with what 
I have described on pages 303 & 304 as occurring in the living Lycopods. I have already 
* See M. Ratvenhoff “ On the Formation of the Cork-bark in Dicotyledons,” Annales des Sciences Naturelles, 
5 i6me serio, vol. xii. p. 34. 
