1920-21.] Size, a Neglected Factor in Stelar Morphology. 15 
this end is attained is by the further advance to polycycly, which is seen 
in beautiful examples in Matonia (fig. 9 : 3), or Pcesia podophylla (fig. 11). 
Three or even four concentric cycles of vascular tissue have been observed, 
while open communication is maintained at or near to the nodes between 
the outer and inner cycles, as well as between the tissues that embed them. 
The proportion of surface to bulk is still further advanced by such means 
as these, which are exemplified in numerous cases 
of ferns not otherwise resembling one another. 
From this it may be concluded that a general 
cause has been at work, which has affected the 
development in distinct phyletic lines. 
The same ends as are gained by solenostely 
are still further promoted by the appearance of 
perforations in the vascular tube. These are 
often very numerous, and are specially found 
in ferns of advanced type, such as Davallia, 
Platycerium (fig. 9 : 7, 8), Polypodium, or Steno- 
chlcena (fig. 12). Each perforation is entirely 
lined by endodermis, which still shuts in the 
vascular tissue completely, while interchange 
between the tissues within and without the tube 
is promoted. The perforated stele may be com- 
pared structurally and physiologically with the 
perforated leaf of Ouvirandra : the problem of 
surface-interchange has been solved in both cases 
by increased surface. The attenuated network of 
vascular tissue which after perforation represents 
the solenostele is characteristic of the most recent fig. 12 —Stenochlo&natmuifolia 
and advanced ferns, and their prevalence in genera 
and species now living is a testimony to the 
physiological success which perforation brings. 
Another step, distinct in time and manner of 
its origin from perforation though resembling it in its effect, is the over- 
lapping of foliar gaps in short stems with crowded leaves. The result is 
what is described as dictyoslely. If a foliar gap underlies each leaf- 
insertion, and the leaves are crowded on the axis, any transverse section 
will cut through more than one of them, and in a transverse section the 
vascular ring will appear divided into a number of isolated tracts. This is 
seen in the Male Fern or in large Tree Ferns (fig. 13, B). It is frequently 
combined with perforation. Though the perforations and the leaf -gaps are 
into a single plane, showing 
perforations. 
l.t. = leaf -trace. 
br. = vascular supply to a branch. 
