578 
DR JOHN M'LEAN THOMPSON ON 
I have quoted these remarks in full so that the reader may see more clearly that 
they are not the bearers of new morphological facts, but are merely the restatement 
of the reduction theory already advocated. But they entail an implication which 
may be briefly considered, namely, that in an organism under xerophytic conditions 
simplification or reduction of the axile stele may be expected to take place, and in 
the case of Platyzoma has actually occurred in the manner indicated above. Such 
a suggestion may gather strength from the facts that the leaves of Platyzoma are 
densely crowded upon the axis, and many of the leaves are of reduced and remark- 
ably modified type (5). The axis is then probably of a condensed and slow-growing 
type, as might be expected under xerophytic conditions. But from this it does not 
seem to follow reasonably that telescoping of the stele has taken place in the 
phyletic sense with the obliteration of foliar gaps. And further, even though it 
were proved that stelar modifications had arisen in Platyzoma under xerophytic 
conditions, no structural evidence has been advanced to show that the stele of 
Platyzoma was at any time solenostelic. Much less has it been shown that in any 
solenostelic fern under xerophytic conditions stelar reduction has taken place and 
has followed the lines indicated, namely, that leaf-gaps have been obliterated and 
inner phloem lost. 
But there is another aspect of this question which must be considered, for it 
lies behind Mr Boodle’s alternative view that “ Platyzoma may have been derived 
from, a protostelic Gleichenia, and its structure might then be due to the formation 
of a pith and internal endodermis.” This suggestion may justifiably be transformed 
so as to be read that the present medullated monostelic condition of Platyzoma is the 
direct result of medullation of an original proto stele which has not acquired leaf -gaps 
during transformation. It is to be granted at once that this alternative upgrade 
view stands, as does the reduction theory, not only “ not proven,” but also so far 
unsupported by facts of observation. It presumes that the pith and internal 
endodermis were created de novo, whereas the reduction theory presumes intrusion 
of the cortex and outer endodermis into the stele through leaf-gaps, the formation 
of a pith of purely cortical origin, and other modifications which culminated in the 
establishment of a typical solenostele ; and when this had been accomplished — 
according to the reduction theory — the leaf-gaps were obliterated and the inner 
phloem completely disappeared. The implication of Mr Boodle’s view of the stele 
of Platyzoma is that the present medullated monostelic state is the high-water 
mark of elaboration from an original protostele reached either under xerophytic 
conditions or independently of them. If this view could be reasonably supported 
by structural facts, it would merit the same consideration as would be due to the 
reduction theory if structural evidence of degenerate internal phloem and of actual 
leaf-gaps were forthcoming. 
The matter stood thus in suspense when in 1908 Dr Jeffrey’s paper entitled 
“ Are there Foliar gaps in the Lycopsida ? ” appeared (10). From it I am extracting 
