35 
Anatomy of the Ophioglossaceae. III. 
internal to the protoxylem of a nascent leaf-trace, by the development of 
parenchyma replacing more or less completely the xylem elements. On the 
other hand, the pith is diminished immediately above a leaf-gap by an 
increased development of inner xylem forming a marked bulge inwards. 
When this rhythm is considered in the light of the stelar structure of 
small rhizomes it gains in significance. Thus in steles with a small pith like 
that shown in Photos 36-39 we find the inner xylem least developed just 
before the leaf-trace begins to separate, while on the separation an increase 
of the inner xylem takes place, which is especially marked on the dorsal 
side of the stele below the gap in the outer xylem. In this case, as in the 
adult rhizome, the pith persists even where the inner xylem is at its greatest 
development, and this is on the dorsal side of the stele below the closing 
leaf-gap. In other cases, and especially in the basal nodes of the young 
plant (Photos 42-46), the inner xylem is less developed as the trace 
prepares to separate, and the subsequent increase of inner xylem leads to 
total filling up the centre of the stele so that the pith is interrupted at the 
base of each internode. 1 Whatever be the ultimate explanation of it, the 
rhythm in proportional development of inner xylem and pith is the same in 
large and small rhizomes. 
The pith can be seen to arise in Helminthostachys by the increase in 
the amount of parenchyma in the centre of the stele. Usually scattered 
elements of parenchyma are present in the inner xylem, though this may 
consist of tracheides only, and all grades between this and the complete 
replacement of inner xylem by parenchyma may be found in juvenile steles. 
The rhythmical variation in amount of the inner xylem appears in some 
cases to negative the idea of the pith being of £ intrusive ’ origin. Thus, in 
such cases as those shown in Photos 35 and 38, the pith is cut off from the 
outside by the inner xylem, 2 the leaf-gap only affecting the outer xylem. 
It is only in the adult rhizomes with larger leaf-gaps that continuity 
between the cortical and medullary parenchyma is marked, and it is in 
these regions that an internal endodermis is often developed. The appear¬ 
ance of this is not suggestive, however, of its being a morphological limit 
between an intrusive cortical pith and the stele ; 3 the occasional presence of 
an internal endodermis in the root is further evidence against attaching 
morphological importance to it in the stem. A still further argument 
against regarding the pith as extra-stelar is afforded by its reduction and 
the distribution of tracheides through it when the stele exhibits condensation 
from the adult to the juvenile type (cf. Photos 51, 52). 
Though the inner xylem differs in its histological features in the 
1 A similar return to a solid stele after a leaf-trace departure is described for a small plant of 
Botrychiwn lunaria by Bower (Ann. of Bot., xxv, p. 541). 
2 Cf. Botrychium lunaria , Ann. of Bot., xxvii, p. 221, PI. XX, Photo 14, and the discussion 
and comparison with Osmunda there. 
s A similar conclusion was reached in the case of B. lunaria. Ibid., p. 217. 
