242 
Lady Isabel Browne. 
These “ nodal islands,” consisting of pouches of phloem enclosed in 
the stele and, sometimes containing an endodermis. extend down¬ 
wards into the internode for a varying distance. Mr. Boodle first 
and Mr. Tansley later have pointed out that the solenostele of G. 
pectinata probably arose by the continuation downwards through the 
internode of an unusually bulky nodal island and its junction with the 
one developed in connection with the node below (5), (33). Mr. Boodle 
points out that the nodal island might be regarded as evidence that 
the protostele has been reduced from a solenostele, but that the 
fact that the young stem is at first protostelic renders this 
improbable. Further the dichotomy of the fronds of many 
Gleicheniaceae renders it quite likely that the order is a relatively 
primitive one. In the xerophytic Platyzoma the departing leaf- 
traces, which are very small and much crowded, leave no gap in the 
stele. Its xerophily and the smallness of its leaves make it very 
probable that it has arisen from a solenostelic form by reduction. 
This view is held both by Mr. Tansley and by Mr. Boodle (5), (33). 
Concerning the form of the leaf there seems no reason to 
doubt the view that, as is usually the case, the dichotomous fronds 
of most species of Gleichenia are more primitive than the simple 
pinnately lobed leaves of G. moniliforme and G. simplex, both of 
which appear to be somewhat reduced. Platyzoma has truly pinnate 
leaves, but this seems to be a secondary modification, a case of 
forking mentioned by Professor Bower favouring this view (7). 
In comparing the Gleicheniaceae with the Botryopterideae we 
find in the simpler members of both orders a solid protostelic 
cylinder. In the Gleicheniaceae the protostele is always mesarch ; 
but the amount of centrifugal xylem present varies in different 
species, and, on the view advanced here, such a species as G. 
vestita, in which the protoxylem is relatively deeply seated, has 
diverged further than most species from the hypothetically exarch 
and protostelic ancestor of Botryopterideae and Gleicheniaceae. 
The dichotomy of most Gleicheniaceous fronds seems to show that 
they are more primitive than any Botryopteridean fronds known to 
us. Moreover the sporangia of the Gleicheniaceae differ so widely 
from those of the Botryopterideae as to forbid our deriving the 
former from the latter order. For example it is impossible to 
regard the annulus of the Gleicheniaceae as homologous with that of 
the annulate Botryopterideae ; both types of annulus must have 
arisen independently from an exannulate sporangium. It then 
appears that the Gleicheniaceae and Botryopterideae have in common 
