9 
1920-21.] Size, a Neglected Factor in Stelar Morphology. 
In other primitive ferns, as a larger size of the stele is attained in the 
growing plant a change of internal structure appears, leading to medulla- 
tion. Since the leaf-traces are inserted peripherally, it is in the outer 
xylem that the water-transit will be most active. As the stele enlarges, 
the water in the central region will tend to stagnate, and thin-walled cells 
Fig. 6. — Outlines of xylem of steles, all drawn to the same scale ( x 5), 
to show approximately relative size. 
i. Botryopteris cylindrica, diameter 'C5 mm. iii. Ankyropteris Grayi, diameter 2 - 5 mm. 
ii. Ankyropteris Grayi, diameter 2’0 mm. iv. Asterochloena laxa, diameter 12*0 mm. 
The elaborateness of outline increases with the size. 
will serve for its storage as well as thick- walled tracheides would do. 
This is probably the rationale of the conditions of “ mixed pith,” and of 
the formation of a parenchymatous medulla. Medullation in one form or 
another is common to the great majority of ferns. Its intra-stelar origin 
has been followed most convincingly for upright stems in the stratigraphical 
sequence of the fossil Osmundacese, described in our Transactions by Kidston 
and Gwynne-Vaughan. It has also been demonstrated in Gleichenia 
pectinata and other ferns by Dr Thompson {Trans. R.S.E vol. Iii, pt. iv, 
p. 715). The parenchymatous pith once established may serve not only 
