230 
PEOEESSOE W. C. WILLIAMSON ON THE OEGANIZATION 
observations which I have recorded place the matter beyond doubt. Such examples as 
figs. 15, 17, 18, & 19, taken in connexion with figs. 5 & 6, make plain what are the por- 
tions of the stem which furnish the appearances so commonly found in fossil examples. 
The only question that is doubtful refers to the way in which the bases of the petioles 
of the fallen fronds have become detached. They evidently withered into membranous 
lamina:, as in some living Cycads and many tree-ferns; but whether they became de- 
tached bodily, leaving a well-defined cicatrix marking their base, as in ordinary deciduous 
trees, or whether the shrivelled stump of the petiole was worn down gradually by atmo- 
spheric decay, as in JEncephalartos caffre and other allied Cycads, is not easy to say. I am 
inclined to conclude that the latter was the true process; but in either case a surface was 
reached, corresponding with the outer surface of the epiderm, at which a well-defined 
cicatrix of parenchymatous cells, of small size and with thickened walls, arrested further 
decomposition. Our knowledge of the relations of fruits to stems is too vague to enable 
us, as yet, to arrive at any definite conclusions respecting those of Halonia ; but if the 
scars which I have referred to in TJlodendron and Halonia really supported cones, they were 
planted upon the subepidermal surface of the outer bark, and, like the leaves and rootlets, 
only received a vascular bundle to supply them with nutriment. What I mean is, that 
there appears to have been no deflection to these scars of any large portion of the vascular 
axis, which would have been the case had these curious organs given origin to branches. 
It appears to me that, connecting the preceding observations with those made in my 
previous memoir on Calamites, we are called upon to make some change in the gene- 
rally accepted views respecting the classification and nomenclature of the living vascular 
Cryptogams. 
To apply the term Acrogens to plants which grew up into magnificent forest trees, 
the structure and growth of whose stems was essentially exogenous, whilst those stems 
exhibited so many of the internal features of exogenous organization, is surely an error. 
Until the close affinities of the Lepidodrendra with the Sigillarise was established by 
actual observation, I do not wonder that M. Brongniart insisted upon his belief that 
the latter were Gymnospermous Exogens. I do not see how this Gymnospermous theory 
can be entertained any longer ; but to make the facts upon which it was based accord 
with our systems we must alter the latter. 
In the discussion which followed the reading of my memoir on Calamites before the 
Boyal Society in January 1871, Dr. Carpenter threw out a suggestion which accords 
with my own conclusions on the question. One great distinction between the Exogens 
and Endogens is to be found in the fact that, when a formation of vessels is made in the 
woody zone of the former type, the clusters of vessels are left uninclosed, and conse- 
quently capable of receiving any amount of addition to their number without inter- 
ference with the continuity of the series. On the other hand, the opposite is the case 
with the Endogens. Here each cluster of vessels is incased in a dense cylinder of woody 
prosenchyma, which latter always interferes to interrupt all continuous additions to the 
former tissues. If we turn to the Cryptogams, especially as illuminated by the study of 
