576 
DR JOHN M‘LEAN THOMPSON ON 
was thus contemplated by Mr Boodle as 'perhaps probable, but an intrastelar origin 
within an original protostele seemed also possible. 
In thus stating the views which seemed alternative, Mr Boodle clearly showed 
that the structural facts — to which he was the chief contributor — were inadequate 
foundations for a confident opinion as to the true origin and nature of the medullated 
stele of Platyzoma. 
Mr Boodle’s work on the anatomy of the Gleicheniacese was followed in 1902 by 
a paper by Dr Jeffrey on “The Structure and Development of the Stem in the 
Pteridophyta and Gymnosperms” (8). It included no new observations on Platy- 
zoma, but the stele was said to closely resemble that of Osmunda cinnamomea, 
since it is a collateral stelar tube with an internal endodermis. The author 
further remarked: “Through the kindness of Dr D. H. Scott he # has had the 
opportunity of examining some sections of a dried stem of this species, f The foliar 
gaps are nearly degenerate in this small extremely xerophytic fern. It would be 
interesting to investigate branching specimens of this species if such occur, to 
discover whether the internal endodermis is continuous with the outer one through 
the ramular gaps. It seems not improbable that this may turn out to be the case.” 
At a later point in the present paper it is shown that although the stem of 
Platyzoma is frequently branched, no ramular gaps exist in the materials examined ; 
the pith has always been found to be completely isolated from the cortex, and no 
connection has been seen between the outer and inner endodermal sheaths. The 
suggestion contained in the concluding remarks of the above quotation may therefore 
be set aside as disproved. But the statement that “ the foliar gaps are nearly 
degenerate ” remains to be considered. It clearly implies that some observation has 
been made from the sections referred to suggesting the presence of reduced or 
degenerate foliar gaps, and, if justified, might aid materially in the interpretation of 
the stele. For it will be evident that the presence of nearly degenerate foliar gaps 
would be strong evidence in support of a belief that the so-called ectophloic 
siphonostele of Platyzoma is the result of reduction, and was at least preceded at 
some point in the ancestry by a siphonostele in which foliar gaps were well defined, 
so that the pith and cortex were in direct connection through them. Dr Scott and 
Mr Boodle have kindly put at my disposal the sections already examined by 
Dr Jeffrey. Each one possesses parenchymatous xylem-rays like those already 
described in the preface of this paper. At several points leaf-traces are departing 
from the stele, and in some instances a parenchymatous ray links the parenchyma 
beneath the leaf-trace-xylem with the parenchymatous zone immediately inside the 
xylem-cylinder. 
It was from such sections as these that Mr Boodle’s analysis of the vascular 
construction was made, and it was probably from one of them that his illustra- 
tions of a departing leaf-trace were taken (figs. 33, 34, pi. xxxix), “ to show the 
* Dr Jeffrey. + i.e. Platyzoma. 
