562 Bower.—On Medullation in the Pteridophyta. 
a weak Botrychium above, p. 548, has its value as bearing on the question 
of origin of the pith in the axis, and is a further indication of the invalidity 
of Professor Jeffrey’s generalization. 
It seems to me useless to bring forward any further considerations in 
order to rebut the rigid assertion of Professor Jeffrey that the pith ‘ must in 
all cases be regarded as a derivative of the cortex \ But criticism without 
construction is arid and unprofitable. Assuming on the basis of such 
examples as those quoted that the pith is not uniform in its origin, I shall 
now suggest a relation which appears to subsist between the form and 
position of the shoot and the type of its medullation, where that structure 
occurs. It is first necessary to show that such a relation exists, and the 
causes of it may be considered later. The two examples above, taken to 
illustrate respectively a stelar and an extrastelar origin of the pith, show 
strong antithesis to one another, both in the position and in the proportions 
of the shoot. The Lepidodendreae were upright, with columnar axis and 
relatively small, closely set leaves, as is also the cone in the living Selagi- 
nella spinulosa. On the other hand, the solenostelic, rhizomatous Ferns, 
such as the Dennstaedtiinae, the Matonineae, and certain Pterideae, are of 
creeping habit, with relatively large leaves, seated singly at intervals along 
an elongated rhizome. In the one case the upright axis is the dominant 
factor in the shoot, and the pith, where present, is intrastelar in origin. In 
the other the leaf is a dominant factor over the weak and prostrate axis, 
and the pith is extrastelar in origin. I suggest that the position of the axis , 
together with the proportions of the appendages to the stem , at the time when 
the medullation was phyletically initiated , have been determining influences 
upon the method of origin of the medullation . Once the method of the 
medullation was stamped in any phylum it might be continued on the 
same plan, with a high degree of conservatism, even though the position of 
the axis or the proportion of the parts might be altered. It is, in fact, 
suggested that in the first instance an upright microphyllous stock has 
favoured intrastelar medullation, while a creeping megaphyllous shoot has 
favoured extrastelar medullation. A test whether this relation is a real 
and valid one will be found in those plants in which mixed conditions occur ; 
for instance, where the axis is and has phyletically been upright, but the 
leaves relatively large ; or, again, where the stock is a creeping one, but the 
leaves only small. It will presently be shown that the mode of origin of 
the pith in cases where such mixed conditions occur accords generally with 
the suggestion, while they supply such various states of the tissues of the 
axis as may lead to a knowledge of the underlying principles which govern 
its structure. 
In Chapter XVI of the ‘ Land Flora ’ I have advanced strong reasons 
for holding that the primitive symmetry of the Archegoniate sporophyte 
was radial. We know also for the Lycopodiales, Equisetal.es, Ophio- 
