572 Bower.—On Medullation in the Pteridophyta. 
bulky to secure stability, and its leaves are crowded. This type of shoot 
favours intrastelar medullation, which is thus biologically related to the 
upright habit. In the creeping forms the mechanical requirements are not 
so urgent, and the axis may be long and relatively thin in proportion to the 
size of the leaves. It may be suggested, further, that the ca?tsa causans of 
the creeping habit itself is the weight of the relatively large leaves which 
the axis cannot itself otherwise support. This then leads to extrastelar 
medullation, which is thus biologically related to the creeping habit. 
According as the balance is struck between extreme conditions, the intru¬ 
sion involves, it may be, only the more superficial tissues of the stele, as in 
the seedlings of Osmunda cinnamomea and weak seedlings of Botrychium ; 
or the inner endodermis, as appears to be the case in the young Marattia ; 
or the extrastelar tissues as well, as in the older Ophioglossaceae, and in 
Leptosporangiate Ferns at large. 
But the most extreme state of involution hitherto recorded is that seen 
in certain Ferns, in which a basket-like structure is assumed. Here the 
involution has extended to the outermost tissues of the axis, and deep 
pockets, lined by the superficial tissue of the stem itself, burrow far into 
the central column of the pith. This has been described by Gwynne- 
Vaughan for Onoclea , Cystopteris, and Aneimict} and it has been again 
referred to by Jeffrey, but without bringing any new instances. 2 A further 
example may be mentioned in Plagiogyria , 3 though here the involutions of 
surface do not extend so deeply as to reach the pith. It may be remarked 
that all these examples come from Ferns which are closely related to forms 
with a creeping habit, and may be understood to have adopted relatively 
recently an upright habit with tufted leaves, and a massive axis for their 
support. Several of them indeed show the prior creeping phase and the 
later tufted state in their individual lives ( Onoclea , Plagiogyria ), while 
others belong to genera containing species with a creeping habit ( Cystopteris , 
Aneimia). It may be that the basket-like structure of the stock in such 
cases results from the inability of the plant to meet the sudden demand of 
material for the construction of a more bulky axis, in species in which the 
adoption of an upright habit has made a broad and massive stock mechani¬ 
cally necessary for the insertion of the crowded leaves. 
From the facts and comparisons above given, which thus lend them¬ 
selves to a reasonable biological interpretation, the final result is that there 
is no rigid law of medullation, by which ‘ the pith must in all cases be 
regarded as a derivative of the cortex ’. The only law appears to be that 
of organic life at large, that organisms accommodate themselves along lines 
of least resistance to their surroundings. That in so accommodating them¬ 
selves their habit is seen to vary, and especially the proportions and attitude 
1 New Phyt., 1905, p. 211. 2 Bot. Gaz., Dec., 19TO. 
3 Ann. of Bot., 1910, p. 429. 
