THE MORPHOLOGY OF THE STELE OF PLATYZOMA MICROPHYLLUM, R. BR, 593 
It is suggested that Platyzoma may provide an illustration of this third and 
apparently exceptional state in which the protostelic structure of the ancestry has 
not yet been definitely restricted to the “ sporeling ” stages but tends to persist in 
the mature organism. If this be so, the structural evidence discussed in these pages 
may indicate a reminiscence of the steps taken in the initial transformation of the 
ancestral protostele, namely, the growth of a pith within the stele itself, and the 
inclusion of the bulk of this intrastelar pith within an independent internal endo- 
dermis created de novo. If this view should prove to be correct when “ sporeling ” 
plants have been examined, and if parallels to it be found in the ontogeny of other 
primitive ferns, the establishment of foliar gaps either late or early in the ontogeny, 
but subsequent to medullation , may reasonably be regarded as merely a further step 
in stelar amplification by which the initially distinct cortex and pith are sooner 
or later directly connected. On such a view the present medullated conductive 
cylinder of Platyzoma would be regarded as the high-water mark of stelar 
amplification so far reached for this plant, neither inner phloem nor stelar gaps 
having been evolved. 
In conclusion, it is held that the recorded facts would clearly allow of the tubular 
medullated stele of Platyzoma being the result of upgrade development from within 
of an original protostele. Nevertheless, the stelar problem cannot be solved until 
the “sporeling” stages have been investigated and solenostely and reduction are 
proved or disproved. 
The author wishes to express his indebtedness to the Carnegie Trust for a grant 
in aid of the illustration of this memoir. 
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(1) Prodr omus Florae, Novas Hollandias et Insulae Van Diemen. 
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(6) Report of the British Association for the Advancement of Science, Toronto , 1897 : Transactions of 
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(14) The Anatomy of Woody Plants, 1917, p. 281.. 
