570 Bower.—On Medullation in the Pteridophyta . 
Ophioglossaceous pith may have been, and probably were of distinct origin, 
and in the first instance have been regularly delimited by endodermal 
sheaths, vestiges of which were first recognized as present in them by Van 
Tieghem and Poirault. But these are more clearly seen in the seedling than 
in the mature state. Thus the ontogeny supports the conclusion. 
In drawing this discussion to a close, its results may be briefly 
summarized as follows :— 
1. The evidence, whether from anatomy of the mature plant, or from 
the development of the seedling with its ontogenetic bearings, or from the 
stratigraphical sequence of the fossils such as the Osmundaceae and Lepido- 
dendraceae, converges in the support of the conclusion that the origin of the 
pith in the Pteridophyta has not been uniform. Its source may vary even 
in the individual according to its stage of development, or its physiological 
condition. 
2. It may be derived in the following ways — 
(i) Wholly from an intraxylic source, by degeneration of the 
vascular tissue: as in the Lepidodendraceae, the primitive 
Osmundaceae, and probably also in the Equisetales :—or partly 
so, as in Selaginella spinulosa , the Ophioglossaceae, the modern 
Osmundaceae when young (Faull), and Marattia (Charles). 
(ii) The conjunctive parenchyma outside the xylem, but still intra- 
stelar, may contribute to it: as in the Ophioglossaceae when 
young, and the seedling Osmnnda (Faull). In Selaginella 
spinulosa also it appears to have played some part. 
(iii) The inner layer of the double endodermis may be a source of 
intrusive parenchyma : this appears to have occurred in the 
Marattia seedling (Charles), and possibly also in some other 
cases. 
(iv) The endodermis and cortex form intrusive leaf-pockets: as in 
the Ophioglossaceae after the first stages of the young plant are 
passed, and in the Leptosporangiate Ferns at large. 
(v) The steles may be adjusted so as to surround a centrally lying 
tract of cortical origin ( Selaginella laevigata , var. Lyallii). 
3. It has been possible to suggest hypothetically certain conditions 
which have been influential in determining from what source the medullation 
shall originate in any given case: viz. that in the first instance an upright 
microphyllous stock has favoured intrastelar medullation, while a creeping 
megaphyllous stock has favoured extrastelar medullation. 
4. It has been found that in upright stocks which are megaphyllous, 
the medullation may be partly intrastelar, partly extrastelar, and that the 
balance between the two factors may be approximately forecast from a 
knowledge of the proportions and position of the shoot. 1 
1 Professor Gwynne-Vaughan suggests as a further factor the form of the leaf-trace itself. 
