602 Ford . — The Anatomy of Psilotum triquetrum. 
resemblance 1 . In his account of the structure of Cheirostrobus in 1897 2 , 
and again later in 1900, Dr. Scott has compared the synangium and its 
axis in the Psilotaceae to the ventral sporangiophore of the Sphenophyllales, 
regarding the vascular supply of the leaf and synangium in Tmesipteris as 
comparable to that found in the bract and sporangiophore of a Sphenophyllum. 
A still closer affinity between the Psilotaceae and the Sphenophyllales was 
suggested by Thomas 3 in 1902, and based largely upon certain variations 
commonly met with in the fertile structures of Tmesipteris. Dichotomy 
may occur in some of the sporophylls, accompanied by an increase in the 
number of synangia ; and this condition, which is also found, but less 
frequently, in Psilotum , is regarded by Thomas as an ancient feature of the 
Psilotaceae. Again, the synangium in Tmesipteris may be raised on a stalk 
or pedicel, and this is compared to the sporangiophore of Bowmanites 
Romeri with its two sporangia. Whether we accept such variations as 
representing normal conditions or otherwise, there nevertheless seems to be 
little doubt that a strong resemblance exists between the Psilotaceae and 
the Sphenophyllales in regard to the sporangial structures. 
Professor Bower 4 has adopted Thomas’s view as to the close relation- 
ship of the two groups, and would include the Psilotaceae and the Spheno- 
phylleae together in the Sphenophyllales. The two series are regarded as 
being related, but as having at the same time two distinct lines of evolution. 
Whilst admitting with Professor Bower the great importance of the spore- 
producing members in plants, nevertheless other considerations must also 
be taken into account. The structure and development of the young 
sporophyte, the anatomy of the mature plant, and again, the nature of the 
gametophyte, are points which must be considered, even though they may 
be ranked as secondary in importance. Dr. Scott 5 has suggested that the 
Psilotaceae may be allied, though very remotely, to the Lycopodieae, having 
perhaps branched off from the main line of Lycopod descent very far back, 
at a point where some of the characters common to the Sphenophyllales 
were still retained ; a knowledge therefore of the gametophyte and of the 
young sporophyte might throw fresh light on this possible relationship. 
As matters stand the affinity of the Psilotaceae with the Sphenophyllales is 
clearly the most marked, but until a further knowledge of the development 
of the living genera of Psilotum and Tmesipteris is known it would seem 
a little premature to assign to them a definite position in a group which 
hitherto has only included fossil forms. 
In regard to the rhizome of Psilotum , a considerable similarity in 
structure has been found to exist with the fossil root or rhizome recently 
described and figured by Professor Weiss 6 . The structure of the two 
1 Boodle (’04), p. 506. 
8 Thomas (’02), p. 342. 
6 Scott (’00), p. 499. 
2 Scott (’97), p. 27 ; (’00), p. 499. 
4 Bower (’03), p. 229. 
6 Weiss (’04), p. 25-5. 
