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XXVIII. — New Stelar Facts, and their Bearing on Stelar Theories for the Ferns. By 
John M‘Lean Thompson, M.A., D.Sc., F.L.S., Lecturer on Plant Morphology, 
Glasgow University. (With Four Plates and Nine Figures in the Text.) 
(MS. received March 1, 1920. Read March 1, 1920. Issued separately July 9, 1920.) 
For over thirty years problems of stelar structure in the Ferns have claimed the 
attention of anatomists. By common consent protostely has been recognised as a 
primitive state. It is seen in all phyla of primitive vascular plants, except the semi- 
aquatic Equisetales, it figures universally in the juvenile plants of the Filicales, and 
is maintained in the adult stems of a number of their primitive genera. 
But as to the origin of pith, inner phloem, and inner endodermis, and the steps 
by which during descent the primitive protostely has in many instances been replaced 
in the adult by other more complex stelar states, there is no general agreement. 
On the one hand, the pith, inner phloem, and inner endodermis have been regarded 
as purely intrastelar tissues directly referable in origin both in individual plants 
and descent to the procambial mass of the growing point of the stem. On this view 
medullation and solenostely are considered consecutive resultants of direct intrastelar 
readjustments, the steps taken being : (i) an ontogenetic progression from the solid 
xylem-core of the protostele in the juvenile axis to a medullated protostele by change 
of destination of procambial elements. These have reached maturity as parenchy- 
matous cells rather than as tracheides ; (ii) a further ontogenetic change from the 
medullated protostele thus established to a solenostele by differentiation of procambial 
cells as sieve-tubes and endodermis within the pith itself ; (iii) the establishment 
of direct histological connection between the intrastelar pith and the cortex through 
gaps in the stelar cylinder. On the other hand, the pith has been interpreted as 
cortical tissue involved in the stele during descent. The pith is considered of purely 
cortical origin, and the inner phloem and inner endodermis of the solenostele are held 
to be extensions of the outer phloem and endodermis of the original protostele. On 
this view the steps of progression from protostely to solenostely are : (i) the solid 
protostele is followed in the ontogeny by a conductive tract into which, from the 
axil of each leaf-trace., a decurrent mass of cortical tissue intrudes. These cortical 
masses are lodged in endodermal pockets ; (ii) as the ontogeny advances succeeding 
pockets of cortical tissue are confluent within the stele, their common product being 
the pith which communicates with the outer cortex through the mouths of the 
pockets or stelar gaps. By extension of phloem and pericycle of the protostele 
during medullation solenostely is attained. 
Both views have been held by mutual exclusion for the Filicales as a whole ; on 
the other hand, pith is considered in some cases of intrastelar origin, in others as 
TRANS. ROY. SOC. EDIN., YOL. LII, PART IV (NO. 28). Ill 
