Worsdell . — The Structure and Origin of the Cycadaceae . 143 
pith becomes much smaller in diameter, the centripetal xylem occupying 
an important part of the area enclosed by the secondary wood. The latter 
tissue at the same level forms a continuous, compact ring in which the 
sinuous character is very much less evident than is the case at a higher level 
of the branch. The result of all this is a much greater resemblance of the 
structure of the branch-base to that of the mature stem of Heteranginm 
than is the case with the structure at a higher level of the branch where the 
typical Lyginodendron-featmes reappear. 
I do not regard this as having any phylogenetic significance, for the 
ontogeny of the structure of a shoot need not necessarily repeat its phylo- 
genetic history. The explanation appears rather to be a mechanical one : 
the concentration, both in a radial and tangential direction, of the woody 
vascular tissues is probably an adaptation for the resistance of tension and 
bending-strains to which the shoot-base must become subject. The same 
adaptation may be observed in Ferns, as in the base of the stem of 
Osmunda , of the branches of Blechnum Spicant, and others. It occurs at 
the base of the stems and branches of Dicotyledons. In the base of the 
peduncle of Stangeria the centrifugal xylem of the ring becomes congested 
in this way, and it is only at a higher level, where the typical structure of 
the organ is assumed, that ancestral characters, such as the presence of the 
centripetal xylem, begin to appear, thus proving that the structure pre- 
vailing at the base of the organ is purely adaptational in character and of 
no phylogenetic significance. It is to the typical mature structure , and not 
to the early stages of its ontogeny, that we must look for the occurrence of 
ancestral characters. The subject cannot be entered upon now, but might 
well repay further investigation. I believe myself that, in all likelihood, 
far too much has been made of the idea that the successive anatomical 
structures exhibited in the ontogenetic history of Ferns and other plants is 
a repetition in parvo of similar structures developed during the course of 
the phylogenetic history. 
Further evidence in support of the view set forth above as to the 
origin of the Lyginodendron- structure is afforded by the early occurrence — 
viz. in the Calciferous Sandstone— of that polystelic member of the same 
group we are considering, Cladoxylon. I may add, as a final corollary to 
this part of the subject, an opinion which I have elsewhere expressed : 
* that the derivation of the tubular from the solid stele is to be sought for, 
not in these semi-gymnospermous forms, but much farther back, viz. among 
the Ferns ! ’ 
In Med. Solmsii and M. Leuckarti (some forms) the two primary 
rings of steles differ in that the steles of the innermost ring are smaller and 
less developed than those of the outer. In M. porosa there is also an inner 
ring which may be said to be formed by the regidar annular grouping of 
certain of the small medullary steles. In fact, transitions exist between 
