Sinnott . — Some Jurassic Osmundaceae from New Zealand . 477 
of the steps in its phylogeny. On the existing evidence it is possible to 
frame theories only, and the question at hand is as to which of the two so 
far in the field is most worthy of adoption by plant anatomists. 
The test of a good hypothesis is the number of facts it will explain. 
Measured by this standard the reduction theory has many advantages over 
the intrastelar. It explains the remarkable structure of O. Skidegatensis 
and related forms. It accounts for the presence of narrow but persistent 
leaf-gaps in the xylem of other species of Osmundites , and of the modern 
Osmundaceae. It accounts for the universal presence of a true parenchyma- 
tous pith in all species of Osmundites and in modern forms, and the absence 
of a ‘ mixed pith ’ or of any intermediate condition between a true pith and 
a solid protostele. It explains the occasional presence in certain living 
species of internal phloem, internal endodermis, and medullary sclerenchyma. 
These facts are either unaccounted for by the intrastelar theory, or 
are explained only by subsidiary hypotheses. The best support for this 
theory is of course furnished by the fossil forms, but there is stronger 
evidence from the same source in favour of the reduction theory. The 
type of O. Skidegatensis is an even more plausible ancestor for the 
Osmundaceae than is Thamnopteris , and has the advantage of being 
connected with living species by a fairly complete series of forms, whereas 
no transitional types are known between the protostelic and the medullated 
Osmundaceae. 
One of the most interesting facts about the Osmundaceae as a whole 
is the extreme slowness with which their structure has been modified. The 
majority of the Jurassic Osmundaceae are very similar anatomically to 
modern forms. The New Zealand species of Osmundites , particularly, are 
almost identical with the members of the living genus Todea , which now 
inhabit the same region. There is perhaps no other case among vascular 
plants where there has been so little change from Mesozoic time to the 
present. 
Summary. 
t. Specimens of Osmundites were obtained from the Jurassic forma- 
tions of the North and South Islands of New Zealand, and are considered 
to belong to the previously described O. Dunlopi . 
3 . They are characterized by a parenchymatous pith, with no internal 
phloem or endodermis. Leaf-gaps are very narrow and often ‘ delayed ’, 
but seem to have been invariably present. 
3. In one specimen typical diarch roots, with a definite cortex, were 
observed in the pith. Roots often enter the pith of Osmunda cinnamomea 
through the branch gap at a dichotomy, and the same thing probably 
happened in the case of our fossil. 
4. The leaf-trace is monarch, and occasionally mesarch at the base. 
K k 3 
