Anatomy of Solenostelic Ferns . 729 
it is clearly shown that the formation of cavity-parenchyma 
is really due to a kind of thylosis. At a certain stage in the 
development of the young petiole the cells of the xylem-sheath 
bordering upon the protoxylem strands increase rapidly in 
size, and send protrusions in between the rings and spirals of 
the first-formed elements, so as partly to fill up their cavities 
(Fig. 29). The protoxylem elements at maturity are almost 
completely disintegrated, and these protrusions give rise to 
the irregularly folded appearance of the longitudinal walls. 
Sometimes, when the leaf-trace protoxylems are decurrent 
down the steles of the stem their cavity-parenchyma also 
accompanies them, as in Dicksonia davallioides (Fig. 27), 
Davallia Novae-Zelandiae , Hypolepis repens , &c. 
A number of irregularly shaped siliceous nodules are to be 
found in the cells of the xylem-parenchyma of Davallia repens> 
D. tenuifolia , D. hymenophylloides , and in most of the Lindsay as 
with the same type of stele. They have exactly the same 
form and appearance as those described by Boodle 1 in 
Lygodium dichotomum. In Davallia repens and Lindsay a 
lohata they also occur in the phloem-parenchyma. 
The first-formed elements of the external phloem are 
always more or less distinct from the rest, and constitute 
a fairly definite protophloem. In most cases a definite 
protophloem is also to be found on that side of the internal 
phloem furthest away from the xylem, but in the Lindsay a- 
type of stele (Figs. 22 and 23) and also in several solenosteles, 
Davallia platyphylla , D. strigosa , D. hirta , Lindsay a retusa , 
&c., no internal protophloem can be made out at all. It has 
already been mentioned that internal phloem is altogether 
absent from the ventral side of the stele in Vittaria slipitata , 
Antrophyum plantagineum , A. recticulatum and A. semico - 
statum. 
The petiolar bundle of the Cyatheaceae and Polypodiaceae 
is essentially concentric throughout. The phloem is most 
plentiful on the abaxial side of the xylem, but at the same 
1 On the anatomy of the Schizaeaceae, Annals of Botany, vol. xiv, no. lviii, 
pp. 364 and 402, 1901. 
3 D 3 
