Maslen . — The Structure of Mesoxylon Sutclijfii (Scott). 383 
thirteen internodes, joining the trace of a leaf vertically below. The 
continuous zone of centrifugal secondary wood is composed of rather large 
pitted elements separated by narrow medullary rays. The secondary 
phloem is excellently preserved in some forms, and in P. Edwardsii it 
consists, between the medullary rays, of alternate tangential bands of large- 
celled parenchyma and of sieve tubes with numerous compound sieve plates 
on their radial walls. The cortex is parenchymatous and contains in its 
outer part a strengthening system of hypodermal strands of sclerenchyma 
of the ordinary Dictyoxylon type, which is so commonly found in the 
stems and petioles of Palaeozoic plants. Periderm formation commenced 
at an early stage, and in older stems in which the phellogen cut right down 
into the phloem the whole of the cortex was thrown off as bark. Each 
leaf received from the stem a single large bilobed bundle (i. e. a double 
leaf-trace) at the base of the petiole, and division of this did not take place 
until the bundles had entered the leaf. 
Since the publication of Bertrand and Renault’s memoir on Poroxylon in 
1886, Renault has added to our knowledge of the genus, 1 and has given an 
account of the phloem of P. Boysseti which shows that the regular structure 
of the phloem of P. Edwardsii mentioned above does not hold for the genus 
as a whole, and so is only a specific character in Poroxylon . 2 
More recently still M. Grand’Eury has identified the leaves of Poroxylon 
in the form of carbonaceous impressions, and finds that they are of great 
size, reaching a length of as much as a metre, with a breadth of 0-15-0-20 metre, 
and gradually narrowing at the base into the petiole. 3 M. Grand’Eury also 
attributes the platyspermic seed known as Rhabdocarpus , Brongniart, to 
Poroxylon : 4 the attribution is on the ground of association, but if it is 
confirmed the evidence of affinity between the Poroxyleae and the Cor- 
daiteae, which has hitherto been based on a study of the vegetative structure, 
will be very considerably strengthened. 
With regard to the affinity of Poroxylon MM. Bertrand and Renault 
regarded the form as related, on the one hand, to the Cordaiteae, and on the 
other to the Sigillarieae. The relation to the Lycopods was supported by 
the case of Sigillariopsis (probably a form of Sigillaria ), which possesses 
double foliar bundles and occasionally pitted tracheides. Bertrand and 
Renault, however, also indicated some of the points of resemblance between 
Poroxylon , Lyginodendron , and Heterangium , in the twin leaf-traces and 
pitted tracheides. 5 In their ‘ conclusions ’ they state that : ‘ Les Poroxylons 
sont done voisins des Sigillariees, des Sigillariopsis , des Lyginodendrons et 
des Heterangium , mais com me les rapports de ces types fossiles avec nos 
1 Renault, Bassin houiller et permien d’Autun et d’Epinac. Flore Fossile, 1896, Etude des 
gites mineraux de la France. 
2 loc. cit., p. 284. 
8 Sur les Rhabdocarpus , les graines et 1 ’evolution des Corda'itees. Comptes rendus, t. cxl, 1905, 
P- 995 - 4 loc. cit. 6 Sur les Poroxylons, loc. cit., pp. 243, 378. 
