of Schizaea malaccana. 
499 
In the first place, we take it, they confirm the idea that the 
normal central parenchyma of the stele of Schizaea is part of 
the system of primitive intra-stelar parenchyma, here forming 
a distinct pith. If we believe, as apparently we must believe, 
that the ‘protostele’ (Jeffrey) with 
solid central xylem, as seen in 
Gleichenia and Lygodium , is the 
primitive type among Ferns, then 
we are naturally led to suppose 
that in Schizaea the central tra- 
cheids of such a stele are nor- 
mally replaced by parenchyma, 
just as is the case in most species 
of Lepidodendron whose structure 
is known. Such a change might 
take place as the result of an in- 
crease in the circumference of 
the whole stele unaccompanied 
by a corresponding increase in 
the total amount of xylem, so 
that the centre of the stele be- 
comes, as it were, vacant as re- 
gards conducting tissue, and is 
filled with parenchyma ; or it 
might happen, and this is more 
probable in the case of Schizaea , 
that while the stele remains of 
the same diameter, or at any 
rate is not increased in size, the 
demand of the plant for water- 
conduction decreases consider- 
ably, and that, as a result, the 
central tracheids are no longer developed, but are replaced by 
parenchyma, among which a few ancestral tracheids still occa- 
sionally appear. The fact that the species of Schizaea are all 
comparatively small plants, mostly living in the humus of deeply 
shaded forests, and with a strikingly small transpiring surface 
M m 
Fig. 23. — Schematic longitudinal 
section of part of an actual stem. 
Endodermis represented by dotted 
lines ; xylem cross-hatched, phloem 
black. 
