Boiver.—On Medullation in the Pteridophyta . 571 
5. It has been found that in a microphyllous stock which is of creeping 
habit an extrastelar medullation may be attained by adjustment of the 
stelar tissue, so as to resemble the condition seen in rhizomatous Ferns. This 
is the case in Selaginella laevigata , var. Lyallii. 
6 . Many facts are found to indicate that when once either intrastelar 
or extrastelar medullation has been initiated in a phylum it is apt to be 
retained, even after a phyletic change of position of the axis has occurred. 
This account of medullation in the Pteridophyta is based upon such data 
as have been quoted in the text. Other factors than those recognized may 
emerge as affecting the origin of the pith, and the summary is open to 
amendment and modification. But it is believed that it more adequately 
expresses the essential features of medullation in the Pteridophytes than 
the rigid statement of Professor Jeffrey that ‘the pith must in all cases be 
regarded as a derivative of the cortex \ 
The work of the future relating to medullation should in the first place 
be to allocate among such sources as those named, or others that may be 
disclosed, the origin of the pith in any specific case; rather than to discuss 
whether or not the pith ‘ must in all cases be regarded as a derivative of the 
cortex’. It is high time that the study of medullation, and of stelar 
morphology, as a whole, should be freed from a form of controversy which 
has in its later phases resembled the contests of scholasticism rather than the 
more elastic discussions of modern biological science. 
Tissues of various character and origin have been seen to contribute, 
either by degradation or by intrusion, to the central parenchymatous column 
commonly found in axes of the more advanced types of structure. We have 
seen that even the pith existing in a single individual may be referred to 
more than one distinct regional source. Its formation may be initiated by 
degradation only, without any intrusion at all, in which case it is strictly intra- 
xylic. Or the formation may be aided, or even initiated by intrusion from 
without: and according as the intrusion is less or more marked it affects 
first the more internal and later successively the outer-lying tissues. It is 
thus seen that the factor of involution from without is only one of those 
which contribute to the general result. In point of fact the pith column, 
with its chief function of storage of water and of nutritive materials, is not 
referable to any single source in the Pteridophyta at large. 
It has been pointed out that the extent of the intrusion of more super¬ 
ficial tissues, whether intrastelar or extrastelar, is closely connected with the 
relation of the leaves to the axis in point of size. Where the axis is bulky 
and the leaves small there is no intrusion ; where the axis is relatively less 
bulky and the leaves large there is apt to be intrusion, and it is more or less 
extensive according to the preponderant size of the leaf is greater or 
less. This balance of the parts of the shoot is, however, itself related to 
habit. An upright axis must for mechanical reasons be relatively short and 
