Psilo turn flaccidum , Wall. 381 
ones pass round the outside of the septum, the middle one passes a short 
way into the septum. In P. flaccidum the central bundle, which passes 
into the middle of the tissue between the three loculi, corresponds to the 
bundle in Tmesipteris that passes into the septum, while no representative 
of the two lateral branches is to be found. 
The structure of the synangium seems otherwise identical with that of 
P. triquetrum , and therefore need not be further discussed here. 
General Considerations. 
From the present investigation it will be seen that Psilotum flaccidum 
closely resembles P. triquetrum in internal structure. In the occasional 
occurrence of very distinct mesarch xylem, in the more general presence 
of leaf-traces, and in the presence of a vascular bundle in the axis of the 
sporangiophore, P. flaccidum approaches Tmesipteris more nearly than does 
the other species, and in these respects serves as a connecting link. 
It is a difficult question to decide which of these three species 
approaches most nearly the ancestral type from which they all sprang. 
The most general opinion is that they are all reduced forms in accordance 
with their epiphytic habit, yet Bower, 1 though he does not dispute this 
reduction theory, on the other hand does not seem inclined to accept it, 
and leaves it an open question, while Lignier 2 regards the Psilotaceae 
as very primitive. 
So far as regards the structure of the Psilotaceae, there is little 
evidence obtainable in support of either view. The feeble development 
of secondary wood in the base of the aerial stem in both P. flaccidum and 
P. triquetrum might be used as an argument in favour of reduction, but 
it might also be regarded as having originated independently in the genus, 
especially as Miss Sykes, 3 in spite of careful search, failed to find any trace 
of it in Tmesipteris. The rootless condition of the plant may also be due 
to reduction, and this seems the more probable when its peculiar epiphytic 
habit is considered. 
The same question of primitiveness or reduction arises when the 
microphylly of Psilotum is considered. Jeffrey 4 and Lignier 6 regard the 
Psilotaceae as primitively microphyllous forms connected with the Lycopo- 
diales, the first-named writer also associating the Equisetales with them. 
The general opinion is, however, that the leaves are reduced structures. 
In this connexion the occasional occurrence of mesarch xylem in both 
P. triquetrum and P. flaccidum may be of importance. Boodle 6 thinks 
that ‘ the most natural conclusion is that the aerial stem of Psilotum has 
1 Bower, F. O. (’ 08 ), p. 413. 
3 Sykes, M. G. (’ 08 ), p. 78. 
5 Lignier, O. (’ 03 ), l.c. 
2 Lignier, O. (’ 03 ), p. 95. 
4 Jeffrey, E. C. (’ 02 ), p. 144. 
6 Boodle, L. A. (’ 04 ), p. 514. 
