Sykes . — The Anatomy and Morphology of Tmesipteris. 77 
trace supplying this leaf arose from the axial strand of the branch just 
before it breaks into three in the ordinary manner. 
3. Lastly, there was the case mentioned above 1 as occurring at the 
apex of the plant of which a continuous series of sections was made. In 
this case a branch, bearing one leaf and a synangium, arose from the main 
stem, which was then terminated by a normal branch. The stele of the 
stem branched into three exactly as does usually the stele of a branch; 
the middle of the three traces supplied the synangium, one of the lateral 
ones ran into the leaf, while the third became the stele of the normal 
branch. 
(i) Ramular and Foliar Gaps. Since Tmesipteris is one of the larger- 
leaved members of the Lycopodiaceous series, the vascular supply of its 
leaves is of interest in relation to the question of leaf-gaps. This interest 
is increased by the fact that the xylem in this plant has a mesarch 
structure. 
Great stress has been laid by Professor Jeffrey 2 on the absence of 
leaf-gaps in the Lycopodiaceae as contrasted with the presence of ramular 
gaps, the absence of leaf-gaps being considered by this author as con- 
stituting an essential difference, of real phylogenetic importance, between 
this phylum and all other vascular plants. 
It has been shown above 3 that while the exit of the ordinary leaf- 
trace from the stele in Tmesipteris leaves no gap in the xylem, the exit 
of the trace supplying the ‘ sporophyll ’ generally leaves a large and obvious 
space. At the same time it has been explained that, though the difference 
between these two methods of development of traces is at first sight very 
considerable, in reality it is only a question of degree, and is due to the 
earlier preparation for the formation of ‘ sporophyll ’ than leaf-traces. If 
we look upon the ‘ sporophyll ’ as a foliar organ, we have here a larger 
leaf than is ordinarily present in the Lycopod series, whose larger vascular 
supply has caused the formation of a gap in the xylem of the stem, 
opposite its place of exit. This gap is not essentially a new production, 
but is mostly due to a difference in degree between the formation of the 
two kinds of traces. 
If, on the other hand, we consider the sporophyll in Tmesipteris to be 
an axial organ, the nature of what may then be looked upon as a ramular 
gap is still of interest, since it shows us how small is really the difference, 
at any rate in this plant, involved in the presence and absence of gaps. 
It would seem that Tmesipteris also throws some light on the meaning 
of mesarch structure. In all the higher plants it is the ‘phanerogamic 
wood ’ 4 or centrifugally developed xylem which gives rise to the leaf- 
traces. In the microphyllous Lycopods the very small and probably reduced 
1 See above, p. 72, and Text-fig. VII. 2 Jeffrey, 1902. 3 See above, pp. 70-1. 
4 Scott, New Phyt., vol. i. 
