THE MORPHOLOGY OF THE STELE OF PLATYZOMA MICROPHYLLUM, R. BR. 577 
nearest approach to a true leaf-gap met with.” In them no leaf-gap affecting the 
endodermis is present, and the only structures which might possibly be interpreted 
as nearly degenerate leaf-gaps are the parenchymatous xylem-rays immediately 
internal to the leaf-trace-xylem. The most obvious ray of this kind which I have 
found in these sections is represented in fig. 26 of the present paper. By them- 
selves such rays can scarcely be held to prove the prior existence of foliar gaps of 
which they are the degenerate relics, and accordingly they cannot reasonably be 
brought into court as proof of the present reduced state of the stele of Platyzoma. 
If such xylem-rays are to be considered the almost degenerate relics of leaf-gaps, 
all prominent rays which completely or almost completely traverse the xylem-ring 
might similarly be considered the degenerate, relics of stelar gaps of one kind or 
another. However true it may prove to be that “ there is ground for the belief that 
the solenostele may give rise by reduction to a modification not distinguishable in 
any way from the medullated monostele of Van Tieghem” (8), the stelar reduction 
indicated above has not been proved for Platyzoma. And further, there will be a 
natural hesitation in accepting as justifiable Dr Jeffrey’s comprehensive statement 
that “ the simplest view, and that most in harmony with all the facts, seems to be 
that the medullated monostelic central cylinder has been derived from an ancestral 
siphonostelic condition with internal phloem by reduction” (8). For the stele of 
Platyzoma is a medullated monostelic cylinder which has not been shown to have 
possessed at any time internal phloem and foliar gaps. 
The reduction theory was supported by Mr Tansley in 1908 (9), and it may be 
well to quote at length his remarks. On page 44 of his Lectures he wrote : “ Platy- 
zoma has a curious and rare type of stele containing a pith surrounded by an 
endodermis but no internal phloem, and with quite small collateral leaf-traces whose 
departure does not break the continuity of the vascular ring as it does in the normal 
solenostelic type.” And on page 106, while discussing the interpretation of the 
Osmundaceous stele, he stated that “ferns which have a continuous internal 
endodermis and no internal phloem are decidedly rare. We have such a case, 
however, in the Grleicheniaceous genus Platyzoma, and the stele of this plant also 
resembles that of the Osmundacese in having no leaf-gaps putting the cortex in 
connection with the pith, the leaf-traces departing from the outer surface of the 
xylem-cylinder somewhat as in the simpler Osmundaceous types. There is good reason 
for considering Platyzoma as an instance of reduction.” On page 108 it was stated : 
“It is no doubt conceivable that Osmunda skidegatensis is, reduced from a normal 
dictyostelic type in which the leaf-traces have secondarily come to depart from the 
outer face of the xylem-cylinder, their internal phloem has ceased to be connected 
with the internal phloem of the stem, and the leaf-gaps have become very narrow. 
Some such process of reduction as this has very likely occurred in Platyzoma.” 
And lastly, on page 134- he wrote : “If the reduction of leaf- traces relatively to the 
stele is great enough, a siphonostelic form like Platyzoma may have no leaf-gaps.” 
