Oct. 19, 1871] ‘ 
NATURE 
491 

tically continuous, in an outward direction, by the 
addition of layer after layer. The materials for the new 
vessels have obviously been furnished by some proto- 
plasmic element which, whether we call it cambium, or 
choose to give it some other name, was located at the line 
of junction between the wood and the bark. The additions 
effected byits agency have gone on through successive ages 
until the thin vascular cylinder became a large hard-wooded 
stem capable of upholding a gigantic forest tree. 
If we turn to the medullary rays, we find that they con- 
sist of vertical laminz of cells. In the tangential section 
they appear asvertical lines of cells (7, Fig. 1, p. 490), undis- 
tinguishable from those seen in the corresponding sections 
of most conifers. In radial sections made in the plane of 
the medullary rays, we find that the latter proceed con- 
tinuously from the pith to the investing bark, with each 
of which tissues they become intimately blended at their 
corresponding extremities. The component cells further 
exhibit, in this radial section, the mural arrangement so | 
characteristic of ordinary medullary rays. As the vascular | 
cylinder increased in diameter by additions to its exterior, 
so these medullary rays became lengthened by the similar 
addition of new cells to their outer extremities, such cells 
being supplied from the same source (cambium) as the 
corresponding new vessels. 
Now, in ail these processes of growth, I re-affirm that we 

have nothing which can, in any plain sense of the word, be 
termed Acrogenous. I can discern no material difference 
between what I have just described and what occurs in 
a Cycad or ina Conzfer. In all these cases the additions 
are equally. made to the exterior of a gradually enlarging 
cylinder, new cells being added to the outer extremities of 
the medullary rays, and vessels to the intermediate lines 
of vascular tissues ; the raw material for both having been 
furnished, as in exogens, by some protoplasmic layer located 
between the vascular cylinder and the bark. I do not 
very clearly understand what Dr. M‘Nab means when he 
speaks ofa“ pseudo-exogenous” growth, or of an “increase 
which takes place in the wood cells of the primitive 
tissues, not, as in Dicotyledons, by additions to the wood- 
cells of the fibro-vascular bundles.” I detect no such 
difference as he seems to imply in the example which I 
have given. 
If I rightly understand his meaning, Dr. M‘Nab 
considers me to affirm that in all these cryptogamic 
plants of the coal-measures, there has been exactly 
the same process of growth, corresponding in each 
minute detail, as takes place during the growth of 
an oak tree. I have never affirmed this. On the con- 
trary, I shall have to show that, amongst these coa 
plants, there are indications of many remarkable combi- 
nations and varied modifications of the process of growth. 

Whether we do or do not accept the doctrine of evolution, 
we should expect to find such generalised combination | 
amongst these primeval forms of vegetable life. I once 
more repeat, however, that these matters are scarcely 
capable of further discussion until my series of detailed 
memoirs has been published. When this takes place, I | 
think Dr. M‘Nab will see that I have not made the two | 
“fatal errors” which he imagines I have done, and that 
there is more in my proposed classification than he, at 
present, has any idea of. Atthe same time I may remind 
him that the recognition of an exogenous process of 
growth amongst cryptogams is not now propounded for 
the first time. Dr. Hofmeister has given us most de- 
tailed accounts of such a process in his history of the de- 
velopment of Isoetes—itself a Lycopodiaceous plant. I 
merely propose to show that a mode of increment which 
now lingers in this one dwarfed genus amongst living 
Lycopodiacez, was once widely diffused, not only through- 
out this group of plants, but equally presented itself 
amongst the Calamitacez. 
Prof. Dyers temperate and intelligent reply to my | 
article on the above subject resolves itself into two 
parts, the first of which deals with facts and the second 
with opinions. As to his facts he is in the same position 
as Dr. M‘Nab. He is not acquainted with the materials 
for forming an opinion which [I have in my hands, and 
upon which my views are based, consequently he has 
taken one extreme type of Lycopodiaceous stem, and 
made its supposed characters representative of the entire 

group. No. 129 of the Proceedings of the Royal Society 
which contains an abstract of my last memoir on the 
subject, would have shown him that I do materially differ 
from Mr. Carruthers in my interpretation of Lefido- 
dendron selaginoides, the plant to. which he refers, 
which difference of opinion I also expressed at the Edin- 
burgh meeting of the British Association. I there showed 
that the central axis does not, as Prof. Dyer affirms, 
“consist wholly of scalariform vessels,” but that these vessels 
are largely intermingled with true scalariform cells. But 
this is not all. The plant in question is but one of a 
large variety of forms. It occupies one end of a linear 
series of types—the opposite extremity of which series 
exhibits a very different aspect. The medullary vessels, 
which, in Lepzdodendron selaginoides, are thus intimately 
commingled with the medullary cellular tissue, in the 
other types gradually recede from the centre to the peri- 
phery of the central cellular axis ; the latter thus assuming 
the condition of a purely cellular parenchymatous pith, 
the cells of which are not even scalariform. The medul- 
lary vessels, thus driven to the periphery, now assume the 
position of the medullary sheath of the higher exogens. 
The vascular tissues for which I claim an exogenous 
origin are superadded to the exterior of this vascular me- 
dullary cylinder. We thus see that the central axis of 
these plants, instead of consisting of two parts, as Prof. 
Dyer affirms, really consists of three,* viz, a central cel- 
* Stigmaria is an exception. In it the medullary vessels are altogether 
absent, as stated in my reply to Dr. M‘Nab. 
