Psilotum flaccidum , Wall. 383 
seems no reason against supposing that these lateral branches have gradually 
replaced an original single central one. Moreover, in Psilotum ordinary 
leaves and sporophylls occur in exactly similar positions with regard to the 
axis, and sporophylls may take the place of leaves on the lower parts 
of the shoot, and leaves may replace sporophylls on the upper part. Again, 
the vascular supply of leaves and sporophylls in P. flaccidum arises in 
exactly the same way from the stele of the main axis. These facts all 
point to the homology of the sporophyll with the ordinary leaf. 
As to the morphological value of the sporangiophore of the Psilotales, 
I share the feeling expressed by H. H. Thomas 1 with regard to this organ 
in the Equisetales, namely, that the evidence derived from the group under 
consideration is not conclusive in favour of any of the theories advanced 
to explain its nature. 
Systematic Position of the Psilotaceae. 
Recent views on the affinities of the Psilotaceae have lately been 
brought together so thoroughly by Lady Isabel Browne, 2 that it will be 
unnecessary to restate them all in detail, though it may perhaps be as well 
to emphasize some points in this connexion. 
For long a relation between the Lycopodiales and Psilotaceae was 
recognized, so that the latter order was included in the former class. This 
resemblance is closest between Lycopodium and Psilotum , both genera having 
exarch protosteles, while lately an occasional occurrence of mesarchy has 
been observed in LycopodiumP as well as in Psilotum. The mode of 
branching is considered as dichotomous in both genera, both are micro- 
phyllous, and in the Psilotaceae the relation of the sporangiophore to 
the sporophyll is similar to that of the sporangium to the sporophyll in 
the Lycopodiales. If Lang’s prothallus referred provisionally to Psilotum* 
actually belongs to that plant, it furnishes additional evidence of this rela¬ 
tionship, for it is very similar to the prothallus of Lycopodium clavatum. 
In 1897 Scott 5 pointed out the close resemblance existing between 
the sporangiophores of the Psilotaceae and those of the Sphenophyllales. 
This resemblance is shared by anatomical characters, for the stele of 
Sphenopliyllum consists generally of a triquetrous exarch protostele with 
three protoxylem-groups, though sometimes each protoxylem-group is 
replaced by two. The normally exarch stele of Psilotum recalls that of 
Sphenophyllum , while the repetition of the triquetrous condition in the base 
of the aerial stem of P. triquetrum, 6 P. flaccidum, and Tmesipteris} and in 
1 Thomas, H. H. (’ 09 ), p. 258. 2 Browne, Lady Isabel (’ 09 ), p. 114. 
3 Sinnott, E. W. (’ 09 ), p. 138. 4 Lang, W. H. (’ 04 ), p. 571. 
5 Scott, D. H. (’ 97 ), p. 27. 6 Boodle, L. A. (’ 04 ), p. 514. 
7 Dangeard, P. A. (’ 91 ), PI. XII, Fig. 10. 
