3i 
Anatomy of the Ophioglossaceae. III. 
studied shows no indication of being intrusive. The pith arises by the greater 
or less development of parenchyma in the central region of the stele. 
There is no indication of conversion of tracheidal elements to parenchyma, 
but this phrase expresses in a general way the mode of origin of the pith, 
since different plants or regions exhibit completely solid inner xylem, inner 
xylem with scattered parenchymatous cells, mixed pith with parenchyma 
predominating in the central region, and lastly a small pith where the 
central region is free from tracheides. It may therefore be stated definitely 
that a pith, such as that shown in PI. Ill, Photos 36, 39, 46, is purely 
intrastelar and in no way due to intrusion. 
None of the seedlings, branches, or juvenile rhizomes of uncertain 
origin showed the actual progression to the fully adult type of stele with an 
internal endodermis, but comparison of rhizomes of different sizes of both 
juvenile and adult type gave a series which lent no support to the view that 
at a later stage of development an intrusive pith was present in addition to 
the primary intrastelar pith. To draw the line on the appearance of an 
internal endodermis would be wholly artificial, as this may be present, 
incomplete, or absent in rhizomes of various sizes. The value of the 
internal endodermis in this plant as an indication of the limit between stele 
and intrusive cortex is further discredited by the occasional appearance of 
an internal endodermis in the base of roots, as described earlier in this 
paper. 
With regard to the change in inner xylem from spiral or reticulate to 
pitted tracheides, the latter characterizing the inner xylem of the adult type 
of rhizome, the evidence of direct transition in any one piece of rhizome is 
also lacking. But it has been shown, in considering the branches and young 
plants, that the inner xylem of the juvenile rhizome behaves like the inner 
metaxylem of the adult rhizome in the region of the leaf-gap, and also that 
in the case of branches there was continuity between the inner xylem of 
the main rhizome and the branch. The question of the transition between 
the two histological types in the ontogeny, though it would be interesting, 
is not vital to the morphological interpretation of the stele. That there is 
a transition at a certain stage may reasonably be assumed. 
Since none of the slender rhizomes of juvenile type, though some of 
them consisted of a considerable number of nodes, showed the advance to 
the adult type, it is clear that this is not necessarily (or perhaps even 
usually) attained quickly. In one piece of rhizome, however, a rapid 
transition from the condition with a stele with a solid xylem to the fully 
adult structure was exhibited, and, though peculiar in some ways, this 
rhizome may be briefly described. It was probably a branch, since, while 
much thicker than any young plant, the first leaf-trace was imperfectly 
lignified and supplied an arrested leaf. The character of the leaf-traces and 
their departure have already been described (see PI. I, Photos 14-19), and 
