OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
297 
direct link between the roots and the leaves. In a large number of plants the 
exogenous zone is merely a superadded structure. In the first instance, as in Lepido- 
dendron Havcourtii, this latter zone is but feebly represented. Nevertheless this is 
a first term in a succession which ends in the same zone assuming: dimensions that 
dwarf the relatively narrow, but still cylindrical medullary, sheath of the plants in 
which the enlarged exogenous growth occurs. In such plants as Sigillaria spinulosa 
and elegans the medullary sheath is no longer an unbroken cylinder. It appears as a 
circle of detached vascular bundles. The exogenous zone now grew more vigorously 
than the medullary sheath, hence the latter was broken up into separated wedges, 
leaving intervening spaces through which a direct cellular communication was esta¬ 
blished between the medulla and the medullary rays of the exogenous zone. Nothing 
of the kind existed in the lower Lepidodendroid forms. In them the medullary 
sheath cut off all such direct communication. These changes bring us very near to 
the narrow medullary sheath, composed, as in living Gymnosperms, of spiral vessels, 
seen in the British Dcidoxylons and in the St. Etienne Cordaites; and, considering how 
many “ missing links ” have been discovered during the last five years, I have little 
doubt but that our continued researches will supply yet further transitional forms, 
and thus establish the unbroken unity of this chain of vegetable structures. 
Hence, though once holding a different view, I am now convinced that so far as we 
are acquainted with the organisation of the plants of the Coal-measures, such facts as I 
have briefly referred to are in thorough conformity with the doctrine of Evolution, 
and that these most characteristic plants can no longer be quoted in opposition to 
that doctrine. 
Our knowledge of the fourth point, viz.: of the organs of reproduction amongst the 
carboniferous plants, is as yet too imperfect to admit of our employing them as wit¬ 
nesses. They teach most definitely the doctrine of the persistence of types. But the 
lack of information respecting alike their autogeny and their phylogeny is amply com¬ 
pensated by what we learn from similar organs amongst their living representatives. 
I would only add, in conclusion, that whilst I believe I have now demonstrated the 
transition from the young Lepidodendroid states of M. Benault’s two groups a and b 
(see page l) up to their matured Sigillarian conditions, there yet remains his third 
type of Sigillarian stem c, of which no Lepidodendroid state has in his opinion 
yet been discovered. He says that such a Sigillaria would require, according to 
my views, a Lepidodendroid representative having the following characteristics 
“ c. Cylindre ligneux forme par une couronne discontinue de faisceaux vasculaires, 
scalariform.es, circonscrivant une moelle depourvue de faisceaux vasculaires ” ( loc . cit., 
p. 247); but such a Lepidodendron M. Benault tells us has not yet been found. 
Yet he announces on page 249 of the same volume that he has found a new 
Lepidodendron of which he has not yet obtained sufficient knowledge to enable him 
to say much about it; but he already sees that he can define its broad features as 
follows :—“Le cylindre ligneux ne serait represente que par une couronne de faisceaux 
