382 Stiles.—The Structure of the Aerial Shoots of 
been reduced from the mesarch to the exarch type in connexion with the 
disappearance of the leaf-traces \ Miss Sykes 1 holds a similar view, 
for she thinks mesarch structure in Tmesipteris and exarch structure in 
Psilotum are to be explained as due to the different size of the leaves 
in the two genera. 
The occurrence of very marked mesarchy in connexion with the 
insertion of a large leaf-trace in P . flaccidum appears to me to lend support 
to this view. 
Boodle also considers that the secondary thickening in Psilotum 
is reduced from the more normal type, and is to be correlated with the 
reduction of the leaves to scales, thus causing a diminution of transpiration. 2 
This writer’s views appear to me to offer the most likely explanation of the 
secondary thickening and occasional mesarchy in Psilotum , but it should 
not be forgotten that these features can be explained as primitive structures, 
and not as due to reduction. 
There remains to be considered the question of the spore-producing 
member. Scott 3 and Bower 4 regard the sporophyll as foliar, the former 
writer considering the sporangiophore in Psilotum as part of the sporophyll 
of which it is an adaxial outgrowth, and which thus occupies a similar 
position to the sporangiophores of the Sphenophyllales. Bower limits the 
term sporophyll to the forked bract, the sporangiophore borne on it being 
non-foliar in nature; it is here associated with the subtending sporophyll, 
but in other groups may be quite independent of it. 
Bower considers the sporangiophore as an organ sui generis . The 
sporophyll he regards as a modified foliage leaf. 
Miss Sykes, 5 adopting the view of Strasburger, Goebel, and Bertrand, 
regards the sporangiophore as a branch of the main axis bearing two leaves 
(the forked ‘ sporophyll ’) and terminating, in the case of Psilotum , in three 
confluent sporangia. Her chief argument in favour of this view is based 
on the fact that in Tmesipteris the bundle in the axis of the sporangio¬ 
phore, after giving off a strand on either side into the lobes of the 
sporophyll, is continued into the septum, a branch being again given off 
on each side which runs round the periphery of the septum. The central 
strand, Miss Sykes thinks, may terminate the axis of the sporangial branch. 
I find myself unable to accept this view. It has been shown that 
a bundle corresponding to the central one in Tmesipteris is also present 
in Psilotum flaccidum , and here it seems simply to serve as a vascular 
supply to the synangium. Miss Sykes herself seems to regard the lateral 
bundles in Tmesipteris as serving the purpose of supplying the large masses 
of developing spores with plentiful supplies of food and water, 0 and there 
1 Sykes, M. G. (’ 08 ), p. 77. 
3 Scott, D. H. (’ 07 ). 
5 Sykes, M. G. (’ 08 ). 
2 Boodle, L. A. (’ 04 ), p. 511. 
4 Bower, F. O. (’ 08 ), p. 426. 
6 Sykes, M. G. (’ 08 ), p. 80. 
