Boodle . — Anatomy of the Schizaeaceae. 407 
sclerotic, in .S', fistulosa it is parenchymatous (or collenchyma- 
tous). There is no internal endodermis in these species ; which 
is perhaps connected with the fact that thore are no inter- 
cellular spaces in the pith. In the next stage in complica- 
tion, internal phloem is present, differentiated from tissue near 
the inner limit of the xylem. This would be formed for the 
attachment of the inner phloem of a concentric or bicollateral 
petiolar bundle. Schizaea having collateral petiolar bundles 
has no internal phloem in the stem. The type with internal 
phloem is not necessarily derived from the medullated type 
with none. If the petiole was already concentric at the time 
when the solid type of stele was departed from, internal phloem 
might be formed as soon as there was any centrally placed 
soft tissue. The one or the other structure might appear first 
in different phyla. No mature Fern appears to be known 
with internal phloem, and at the same time no inner endo- 
dermis, but this condition is met with in the seedling stem 
of Anemia. This is where there is not much tissue within 
the phloem ; but, when a large mass of parenchyma with 
intercellular spaces is present, an inner endodermis becomes 
necessary. This is the solenostelic stage. This central tissue 
is not required by the stele, and for it to be of use, e. g. in 
storing up starch, it must be in free connexion with the outer 
ground-tissue; hence (as all the walls of endodermal cells 
frequently become suberized) the necessity for leaf-gaps as 
found in the solenostele. Continuity being established 
between the central and outer ground-tissue, they may have 
the same functions and resemble one another structurally. 
Nothing further need be said as to the dialystelic type, which 
only differs from the solenostelic in its more crowded leaf-gaps. 
The preceding is a purely theoretical discussion, and of 
course proves nothing as to the correctness of the hypothesis 
with which we started. One may now see whether anything 
can be done towards solving the morphological problem. 
The structure of the seedling is accepted as giving some 
clue to the course of phylogenetic changes. In describing 
the seedling of Pteris aquilina , Jeffrey (’00, p. 9) points out 
