13 
1920-21.] Size, a Neglected Factor in Stelar Morphology. 
where the transition is effected from the protostele. Medullation may 
precede this, as it does in Gleichenia pectinata. In others no previous 
medullation may be seen. In either case a condition of physiological 
success appears in the Leptosporangiate Ferns to be the continuity of the 
sheath, so as to allow no leakage. In the Eusporangiates, as already 
explained, this appears to be less important. 
The chief steps in the advancing complexity of the vascular system of 
Fig. 9. — Series of solenostelic and dictyostelic stems of Ferns, all drawn 
to same scale. ( x 2. ) 
1 , Metaxya ; 2 , Dipteris conjugata ; 3 , Mat onia pectinata ; 4 , Plagiogyria pycnophylla ; 5 , Thyrsopteris 
elegans ; 6 , Saccoloma elegans ; 7 , Platycerium alcicorne ; 8 , Platy cerium cethiopicum. 
These drawings show that the disintegration of the stele does not depend 
on absolute size alone. 
the Leptosporangiate Ferns are known as solenostely, polycycly, per- 
foration, and dictyostely . Such advances may be traced either in the 
ontogeny, or in the race by comparison of distinct species or genera. They 
all result in increase of surface in proportion to bulk of the stelar tissue, 
and in all of them the endodermal barriers are strictly maintained, while 
intercellular spaces are consistently absent from the vascular tissues. They 
all follow on a very considerable increase in size of the system as a whole 
and are believed to be causally related to it. In solenostely the solid 
protostele is replaced by a hollow tube lined within and without by con- 
tinuous endodermis (fig. 9:1,2). At each leaf-insertion a foliar gap leads 
