Afji'nities of Helminthostachys zeylanica. 425 
are found to lie in a cavity in the petiole of an older one. 
Ultimately, when a leaf unfolds, it ruptures the sheath 
provided for it by the base and petiole of the older leaf. 
The same arrangement in all essentials is met with in 
Helminthostachys , and the chief points of difference are such 
as are readily referable to its dorsiventral habit as opposed to 
the radial structure characteristic of Botrychium . Each leaf 
as it arises becomes very early differentiated into a basal 
portion and an upper part, at first represented by the blade 
and sporangial regions, the petiole only becoming visible as 
an intercalated structure at a much later epoch. A broad 
flap grows out from the adaxial face of the leaf-base, and it 
has a broad insertion on the stem also, there being direct 
continuity of the stem- and flap-tissues on the edges near the 
leaf, and thus the stipule comes to partially envelop the stem, 
leaving however a slit-like orifice into its cavity at the apical 
end, exactly as in the case with Botrychium. The apical edge 
of the flap grows completely over the stem-apex and meets 
the tissues of the ventral face of the rhizome on the other side 
of the growing-point. This is facilitated by the fact that the 
apex is sunk in a depression and thus is arched over by older 
tissues on all sides. The Fig. 4 on PI. XXI clearly explains 
these relationships of position. 
As a still younger leaf arises, it also produces a similar 
outgrowth under the flap belonging to the next older leaf; 
hence the apex is enclosed in a series of hoods, separated 
from each other by narrow air-spaces. One result of this 
arrangement is that of ensuring protection for the young 
leaves and the apex of the rhizome from desiccation, whilst 
at the same time the growing tissues are in direct communica- 
tion, though only by means of tortuous passages, with the 
external air. The risk of drought is further lessened not only 
by the fact that the apertures left between the series of flaps 
and the stem-tissues are all on the lower side, i.e . next the 
soil, but also by the presence of mucilage-forming hairs 
which spring from the ends of the flaps and grow into the 
chambers. These hairs are similar to those found in Botry - 
