504 Tansley and JLulham. — A Study of the 
mesarch spiral protoxylem continuous with that of the leaf ( D . rubiginosa , 
davallioides , adiantoides ), exarch non-spiral protoxylem (D. apiifolia ), and 
irregularity of xylem differentiation (D. Barometz) l . These facts, taken in 
conjunction with those obtaining in Trichomanes 2 , are sufficient to show 
that the existence of spiral protoxylem and even of localized protoxylem 
in any form is a highly variable phenomenon as between species and species 
in certain genera of Ferns, in striking contrast to its constancy in many other 
vascular plants. It is highly probable that a careful developmental study 
of the relations would generally reveal specific adaptation. 
In Matonia pectin ata, the distribution of the spiral protoxylems is 
of some interest, and so far as it goes certainly tends to show that the 
development of these structures is strictly dependent on the elongation of 
the stem after the beginning of tracheal differentiation. 
In none of the six young plants A to F, nor in the weaker starved 
plant (type G), can spiral protoxylem be detected in rhizome or petiole. 
We cannot state that no localized protoxylem of any kind occurs, because 
we have not succeeded in finding a good case of partly differentiated xylem 
in stele or leaf-trace. Apical growth appears to be very slow, and the 
complete differentiation of the vascular tissues extends to within a very 
short distance of the growing point. In the type of leaf-trace characteristic 
of stages IV to VI (see table, p. 498) the xylem usually consists of a central 
(abaxial) strand of narrow tracheids and two lateral wings of wider tracheids 
(PI. XXXII, Fig. 22). The central strand is in some cases very clearly distinct 
from the wings. It seems probable that this central strand may be formed 
before the wings, and thus represent protoxylem, but we have not succeeded 
in establishing spiral elements ; in most cases the narrow tracheids are 
certainly scalariform. As the arch of the trace becomes broader (VII and 
VIII) the tracheids tend to become grouped in a series of curved bands as 
in the adult type, and small groups of narrow tracheids are distinguishable 
between the bands (Fig. 23), i.e. in the position of protoxylems in the 
adult ; but these are not constant. 
In stage IX (H), however, a plant which, it will be remembered, had 
a much stouter rhizome and much longer petioles than any of the simpler 
ones, we meet with undoubted spiral protoxylems, both in the rhizome 
and petiole. The stelar system here consists of a solenostele, with an 
internal accessory solenostelic cylinder (Text-Fig. 14, A) supplying the 
lateral loops of the leaf-trace. The external cylinder has its xylem 
arranged in not very well defined arcs, like those of the adult (Seward 
(’ 99 ), Plate XVII, Figs. 8 and 9), with unmistakable spiral protoxylems 
(eight or nine in all) at their points of junction. On the ventral side 
these arcs are longer and broader than on the dorsal, so that the cylinder 
of xylem is thinner and has more protoxylems on the dorsal side, which 
1 Gwynne- Vaughan (’03), p. 727 . 2 Boodle (’00). 
