19 
1920-21.] Size, a Neglected Factor in Stelar Morphology. 
semicordata. The slits are in fact perforations of essentially the same 
nature as those which occur in the axis, and their function is the same. 
They are most prominently 
seen in leaves of large size, 
and especially where the 
margins of the curve ap- 
proach one another, as in 
Thyrsopteris or Alsophila 
(fig. 17 : 5, 6). It may. 
Fig. 16. — Transverse section of petiole of Athyrium then, be concluded that in 
filix fcemina, ferns of advanced type the 
(a) Near base ; (&) higher up. After Luerssen. (x7.) . „ . , . 
leat-trace is subject to per- 
foration in essentially the same way, and with the same physiological effect, 
as is the stele of their axes. 
The stele in the root of ferns is always small, so that no difficult problem 
of proportion of surface to bulk arises. The general tendency to concentrate 
the vascular tissues at the centre of the transverse section of the root, which 
is so general in the Pterido- 
phytes at large, leads to the 
same result. Even in the 
large roots of the Marattiacese 
there is no need for any dis- 
ruption of the compact stele. 
The triumph of the Lepto- 
sporangiate Ferns which show 
disintegration of the stele in 
stem and leaf in all their more 
advanced types, is witnessed 
by their 6000 living species 
over the face 
of the earth. That triumph 
has been won by a compro- 
mise, effected without cambial 
increase in the enlarging stem. The conducting stele has enlarged with 
the conically enlarging shoot: it has maintained its endodermal barriers 
complete, and has met the difficulty of physiological interchange conse- 
quent on that enlargement by various steps of moulding and disintegration 
of the stele. This has given those conducting tracts the requisite propor- 
tion of surface to bulk even for stems as large as those of the Tree Ferns. 
The analogy between what is seen in ferns and the “ polystelic ” state 
Fig. 17. — Transverse sections of petioles, all drawn 
to the same scale. ( x 2. ) 
1, Dipteris conjugate/, ; 2, Dipteris Lobbiana ; 3, Metaxya ; 
4, Phlebodium aureum ; 5, Thyrsopteris ; 6, Alsophila australis. 
These show that while greater size leads to vascular 
disintegration, there is no definite proportion. 
