OF THE FOSSIL PLANTS OF THE COAL-MEASUEES. 
307 
further says, “ das echte Cambium der Dicotyledon dagegen erzeugt sowohl nach aussen 
als nach innen fibro-yasale Gebilde, nach aussen Phloem, nach innen Xylern” (Joe. cit. 
p. 397). If this determination that a cambium-layer must develop tissues on both its 
inner and outer surfaces is to be accepted, there is no further room for discussing the 
matter. We shall see directly that I find no reasons for believing that the bark increased 
its inner surface by prosenchymatous additions from a true cambium-layer, and we have 
nothing in the interiors of these stems corresponding with the ordinary wood-cells of the 
Dicotyledons and Coniferse. I have never for a moment pretended that we find in these 
arborescent Cryptogams all the features of a highly developed exogenous Dicotyledon. 
Primarily seeking to show the absurdity of applying the term acrogens to these plants, 
I have done so by demonstrating that they grow by the addition of new layers to the 
periphery of the old ones, that their woody wedges are disposed in radiating laminse, as 
in the Coniferae, and that these laminae are separated by medullary rays of which the 
cells exhibit a mural arrangement. Whatever name may be given to the genetic mate- 
rial out of which these new investing layers develop, whether we choose to term it 
cambium or meristem, we have here very manifestly a form of exogenous growth. 
That this exogenous structure belongs, as Professor King long ago pointed out, to a 
system of vessels wholly independent of and distinct from the medullary cylinder is 
clear. What its functions may be is not equally clear. It undoubtedly gave strength 
to the trunk and branches of the tree, but it contributed nothing directly to the nutrition 
of the leaves. The leaf-bundles pass through it, but they clearly have no further 
connexion with it than results from that positional relationship. When I wrote the 
second memoir of this series I had not ascertained so clearly as I have since done the 
relations of these foliar bundles. Two facts, however, require further notice. One is that 
in that memoir I described a unique bit of a Diploxylon- stem in which some vascular 
bundles were given off from the ligneous zone, but whether or not they were foliar I 
cannot say*. The other relates to Stigmaria. That this is the root of a Lepidodendroid 
plant is unquestionable. It is also well known that the vascular medullary cylinder is 
not represented in it. The pith, which is large, is in direct contact with the inner surface 
of the exogenous woody zone. Remembering the apparent origin of the medullary 
cylinder from the leaf-bundles, we can understand the possibility that the downward 
prolongations of them would not reach the roots. But, as I have illustrated in my last 
memoir, the exogenous woody axis of Stigmaria does give off the vascular bundles to the 
rootlets. Hence it would appear that the nutritive fluids were absorbed by the rootlets 
and transmitted up the stem primarily by the vessels of the exogenous zone ; but in 
order that those fluids should reach the leaves, they had to be transferred, by some lateral 
movement, to the vessels of the medullary cylinder. I do not propound this otherwise 
than as an hypothesis ; but I can see no other way in which the end could be attained. 
* I think it more than probable that this curious specimen may belong to that part of the base of the stem 
where the medullary vascular cylinder of the latter and the woody zones of the roots with their peculiar Stig- 
marian structure somewhat overlap one another. 
2 T 
MDCCCLXXII. 
