134 WorsdelL— T he Structure arid Origin of the Cycadaceoe . 
M. stellata , Cotta, var. iypica , exhibits also in the region between the 
places of insertion of the leaf-trace bundles a continuous zone in transverse 
section, but here the inner xylem and phloem are very poorly developed as 
compared with the outer. 
M. stellata , Cotta, var. gig ante a ^ shows the same character. But here 
there occur in addition two or three extra cylinders from which all traces 
of the inner xylem and phloem (which may or may not have existed in the 
ancestor of the plant) are completely absent, so that these zones possess 
a collateral structure. They appear also to be entirely secondary in origin, 
for there are no signs of primary tracheides. 
In M. Leuckarti , Gopp. and Stenz., there is a main cylinder composed 
of two or three very irregular, sinuous, elongated steles of considerable 
development ; within this is a minor cylinder of similarly-shaped, but 
smaller steles, which abstricts off at places the smaller steles which com- 
pose the medullary system. 
In M.porosa, Cotta, and M. Solmsii , Schenk, there is likewise an inner 
minor cylinder, but composed, in the first-named species, as also in 
M. Solmsii , Schenk, vars. incrassata and lignosa , of steles greatly reduced 
in size as compared with the main cylinder, forming in this respect 
a transition to the medullary steles lying irregularly scattered in the 
centre. They exhibit, moreover, the peculiarity of having the secondary 
xylem and phloem developed on the inner side only, and are therefore 
collateral in structure. 
The foliage now known to have been borne by Medullosean-stems was 
that known as Alethopteris and Neuropteris 1 , the characters of which are 
very Fern-like. 
The petioles of these fronds, known as Myeloxylon , possess, as seen in 
transverse section, an outer cortical region exhibiting the ‘ Dictyoxylon 5 
sclerotic tissue ; the rest of the organ contains large numbers of collateral, 
usually exarch bundles, orientated in every conceivable direction. 
We will now describe the main features of the anatomy of another type 
of the Pteridosperms, viz. Lyginodendron. A transverse section of the stem 
of L . Oldhamium reveals the following structure: an outer ‘Dictyoxylon’ 
cortex ; an inner soft-celled cortex ; a periderm, probably arising from the 
pericycle ; a central cylinder composed (frequently) of a number of more or 
less distinct and separate bundles each of which possesses the following 
structure : a mesarch primary xylem-strand, consisting of a central proto- 
xylem with, on the inner side, a well-developed centripetal, and, on the outer 
side, a small amount of centrifugal xylem ; attached to the outer side of 
each such primary strand is a thick fan-shaped mass of secondary xylem, 
bounded on its external side by a very much less-developed zone of 
phloem. Opposite each gap between the bundles a single large leaf-trace, 
1 Probably also other types, such as Odontopteris and Linopteris , belonged to them. 
