THE MORPHOLOGY OF THE STELE OF PLATYZOMA MICRQPH YLLUM, R. BR. 575 
decurrent sclerenchyma is gradually surrounded by tbe leaf- trace as the latter is 
followed downwards, so that a deep sclerotic pocket is enclosed by the tubular portion 
of the trace and appears, in transverse section, completely isolated from the cortex. 
“ If now,” he continues, “ we imagine that, the length of the internodes diminishing, 
the leaves of a Eugleichenia approach each other closely, the base of each one of 
these being the seat of formation of a pocket, one can conceive the coalescence of 
these closely meeting' formations leading to a general inclusion which will be enclosed 
in the stem like that seen in Gleichenia polypoides, and the resulting structure will 
be very near to that seen in the stem of Platyzoma.” * 
It will be apparent that Dr Poirault imagined certain non-existing conditions 
as fulfilled in a Eugleichenia , and that he merely contemplated a possible explana- 
tion for Platyzoma , for which no adequate supporting evidence was provided. 
This idea of the origin of the pith of Platyzoma was taken up in 1897 by 
Dr Jeffrey (6). On page 869 of the report of the British Association meeting it 
is stated : — “ Among the Gleicheniacese we have in Martensia the cortex sending 
parenchymatous strands into the vascular axis of the stem down through the 
channelled leaf-traces. In Gleichenia and Platyzoma these are completely cut off 
from the outside cortex, and we have a completely included pith similar to that 
present in Osmunda. The pith in these forms is in reality extrastelar, but no longer 
communicates with the peripheral cortex.” Thus a possible explanation advanced 
by Dr Poirault for the stele of Platyzoma was transformed by Dr Jeffrey into 
the true and actual explanation, even although no structural evidence was advanced 
which would justify dogmatism. For it will be noted that a direct ground-tissue- 
connection between the cortex and stelar pith of Platyzoma has not been recorded 
by any investigator, and accordingly the cutting of such a connection has not been 
demonstrated even in the phyletic sense. 
In 1900 (7) the extremity of this position was modified, though indeed no further 
facts had emerged regarding this plant. Thus on page 7 of this paper it was stated : 
“ It has further been rendered probable by the interesting investigations of Poirault 
that the apparently medullated monostelic central cylinder of the stem of the 
Gleicheniaceous genus Platyzoma also possesses a pith derived from the extrastelar 
fundamental tissue.” 
In 1901 (4) Mr Boodle, after personal observation of the stelar structure, wrote 
as follows : — “ Platyzoma appears to be a xerophytically reduced form in which the 
leaf-traces have become small and crowded ; it is perhaps probable that it may have 
been derived from a solenostelic form, by obliteration of the leaf-gaps and disappear- 
ance of the internal phloem of the stele. This view is, I believe, held by Dr E. C. 
Jeffrey. It is possible, on the other hand, 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.” An extrastelar origin for the pith of Platyzoma 
* The italics throughout this paper are those of the writer of this memoir. 
