152 Gibson . — Contributions towards a Knowledge of 
spiral tracheides, and in section (e.g. of S. Braunii) may 
enclose the spiral elements completely. In all cases which 
I have examined the vein is much more delicate nearer 
the base of the leaf, and expands at its apex. In the very 
young leaf of 5. Kraussiana the reticulate elements at the apex 
may be fully developed before the differentiation of the 
characteristic markings on the spiral and annular tracheides. 
The xylem is partly surrounded by a few long and narrow 
parenchyma-cells, and one or two sieve-tubes with charac- 
teristic plates are present if the bundle be at all massive. 
In many cases I was quite unable to convince myself that 
genuine sieve-tubes were present. All the tubes investing the 
xylem are elongated, thin-walled, and contain nuclei and 
protoplasm. These again are surrounded by a layer of larger 
cells without intercellular spaces, usually containing a small 
amount of chlorophyll, which may be considered as a 
peridesm. I find no evidence of the existence of a genuine 
endodermis, save just at the base of the leaf, where cells 
comparable to and in direct continuity with the endodermal 
cells of the stele occur. The midrib of A. Lyallii is somewhat 1 
irregular in character. The xylem-elements are arranged in 
three patches ; the central strand alone has genuine sieve-tubes 
on its aligular face, but each of the three strands is surrounded 
by parenchyma, and all three are enclosed in a definite layer 
(probably peridesm), abutting on reticulate mesophyll below 
and palisade-cells above (Fig. 17 ). 
In a previous paper 1 I called attention to certain types of 
stem-structure in the genus Setaginella , and made a comparison 
between the grouping of species based on such anatomical 
characters and that usually accepted by systematists. Con- 
sidering the anatomy of the stem only, the majority of species 
examined may be grouped round A. Martensii , all charac- 
terized by the presence of a single stele and by dorsiventrality 
both of external morphological features and internal anatomy. 
S. uncinata formed a transition-type to that of 5. inaequali- 
1 On the diagnostic characters of the subgenera and species of Selaginella: 
Trans. Biol. Soc. Liverpool, 1895. 
