5 6 ; 
Bower .— On Medullation in the Pteridophyta. 
Selaginella laevigata , var. Lyallii} has a short creeping rhizome, 
sharply differentiated from the upright { foliage ’ shoots. The latter show 
a number of steles arranged with no definite order. But in the creeping 
rhizome the stelar arrangement closely resembles that of a typical Filicinean 
solenostele, with gaps formed here not by the departure of leaf-traces, but 
of the aerial branch-traces. Centrally there may be an accessory vascular 
strand, or more than one, behaving just as do the central strands in certain 
rhizomes of solenostelic Ferns. The similarity is so great that it has re¬ 
peatedly been the subject of remark, and it has even been suggested that it 
might have a phylogenetic bearing. 1 2 This would seem to be an erroneous 
view, for S. laevigata is one of the more highly specialized and most 
definitely rhizomatous species of the genus, in which its type of structure 
is rare, if not actually unique. Moreover, the structural comparison is 
with such Ferns as the Pterideae and Matonineae, which are not specially 
primitive types among the Leptosporangiates, and phyletic comparisons can 
only properly be suggested between the most primitive types of such distant 
phyla, if indeed at all. It seems to be an example, and a very remarkable 
one, of homoplasy, of which the stimulating cause has been the horizontal 
position of the rhizome in each case. But the stelar gaps in the rhizomatous 
Ferns are foliar, in Selaginella laevigata they are ramular. In both cases 
the inserted organ is relatively large as compared with the rhizome itself, 
and this proportion has probably been also a factor leading towards the 
similar structural result. The existence of such a structure in a micro- 
phyllous type with creeping rhizome provides an example in accordance 
with our second theoretical anticipation. This plant, in accordance with 
the creeping habit of the rhizome, has by adjustment of the stelar masses 
enclosed a pith-like mass of ground tissue, which was plainly of extrastelar 
origin. 
The structure thus seen in these converse test cases indicates that the 
suggested relation is really existent. It appears that the position of the 
axis and the proportion of the appendages relatively to the axis have been 
determining factors in the formation of the pith respectively from an extra- 
or an intrastelar source. It will, however, be at once objected that the 
Dicksonieae and the Cyatheae, and a host of other Leptosporangiate Ferns, 
are megaphyllous, have upright axes, and still their pith is of extrastelar 
origin. At first this would seem to be a fatal objection. But it is to be 
remembered that according to the theory the determining influence is effective 
at the time when the medullation is first initiated , and once started , the same 
type of medullation is apt to be retained , even after a change of position of the 
axis has occurred. At the British Association at Sheffield (1910) I brought 
1 See Harvey Gibson, Ann. Bot, viii,. p. 187, PI. IX, Fig. 76, and PI. X, Figs. 77, 83-93 ; 
also Jeffrey, Mem. Bost. Soc. Nat. Hist., vol. v, No. 5, p. 160; also Tansley, Lectures, p. 135. 
2 Harvey Gibson, 1 . c., p. 192. 
