Psilo turn flciccidum , Wall. 
385 
the two groups are connected so far back that the gap between them is 
about as great a one as exists between any two groups of the Pteridophyta. 
The arguments against Lignier’s view have been put forward very clearly 
by Lady Isabel Browne, 1 and it is unnecessary to repeat them here. The 
suggestion of an axillary mode of branching in the Psilotaceae might, 
however, add another objection to the list, and the resemblance between the 
Psilotaceae and Sphenophyllales appear too pronounced to warrant their 
distant separation. 
It seems to me quite possible that the Psilotaceae may connect the 
Sphenophyllales and Lycopodiales, while at the same time Archaeopteris 
may be a connecting link between the Sphenophyllales and the Ferns. It 
should, however, be noted that Kidston 2 regards the Archaeopterideae as 
Pteridosperms and not P'erns at all. Moreover, it must be admitted that 
our knowledge of Archaeopteris is so slight that this form can at present 
be of little use in phylogenetic considerations. 
In any case, whether the Psilotaceae are reduced or primitive, the 
evidence at present available seems to indicate a relationship between these 
forms and the Sphenophyllales, and also, though less marked, with the 
Lycopodiales. 
Summary. 
1. In external appearance the aerial stems of Psilotum flaccidmn differ 
from those of P. triquetrum in being rounded below, triquetrous above this, 
and ultimately flattened. In the triquetrous region branching probably 
takes place in planes successively at right angles; in the flattened part 
branching is all in one plane. A leaf seems to be constantly associated 
with stem bifurcation. 
2. The stem-stele is band-shaped in the lowest part of the aerial shoot, 
but becomes triquetrous. When the stem is triquetrous the xylem-mass is 
typically hexarch; in the flattened parts the stele changes from a hexarch 
to a tetrarch condition. 
3. The leaves often receive a vascular supply, and when this is the 
case with a leaf below the stem fork, the leaf-trace is given off so that one 
branch of the stele is practically in the axil of the leaf-trace. 
4. Secondary thickening similar to that found in P. triquetrum is also 
found in P. flaccidum. 
5. Mesarch structure occurs occasionally in the lower part of the aerial 
stem, sometimes in connexion with the leaf-traces. 
6. The sporangiophore-trace is given off in the same manner as a leaf- 
trace. It is continued into the sporangiophore, and terminates in the 
central tissue between the three confluent sporangia. It is thus similar in 
position to the median bundle in the synangium of Tmesipteris. 
1 Browne, Lady Isabel (’ 09 ), p. 115. 2 Kidston, R. (’ 06 ), p. 434. 
c c 
