600 Ford . — The Anatomy of P si to turn triquetrum. 
been made to germinate spores of Psilotum , but without success. The 
spores have been sown in ordinary soil, in sterilised soil and sand, with 
varying conditions of heat, light, and moisture, at the base and in the 
crevices of tree and fern trunks in the tropical houses at Cambridge. 
Spores also have been sown in hanging-drop cultures, in water, and in 
different solutions of salts. Cultivations of the Fungus have also been 
made, and either the soil in which the spores were sown or the spores 
themselves were infected with portions of Fungus, but with no success. 
At the time of writing more attempts are being made under different 
conditions, though it seems improbable that any favourable results 
will follow. 
Theoretical Conclusions. 
Psilotum is generally regarded as representing an ancient type, which 
has retained to a certain extent some of its primitive features, whilst 
showing at the same time modifications due to its manner of life. 
Mr. Boodle 1 considers that the secondary tracheids in the stem probably 
represent a more normal secondary thickening, the reduction being in 
correlation with that of the leaves. Prof. Lignier 2 , however, regards 
the Psiloteae as a very ancient type, this being shown by the primitive 
nature of the leaves. 
In relation to the foliar structure the question as to the origin of the 
pith in Psilotum is of interest. If the small scale-like leaves with no 
vascular supply are to be regarded as primitive, then there is no doubt 
as to the stelar nature of the pith, for unlike other Pteridophyta there 
are no leaf traces to be considered, and the branching is not altogether 
constant at this level and cannot be taken into account. There are, 
therefore, no obvious points at which any continuity can be traced 
and any intrusion of the cortical into the stelar tissues can be observed. 
The protostele in the lower part of the stem is therefore succeeded by 
a medullated condition, and no further stage is found except the sub- 
stitution, for a purely mechanical reason, of sclerenchymatous fibres for 
the parenchyma in the aerial stem. As each aerial branch from the 
rhizome turns to grow vertically upwards above the soil, cortical as well 
as stelar tissues undergo considerable change. More mechanical support 
is needed by the plant, and this for a short time is undertaken by the 
whole brown-walled cortex. Later the stele increases somewhat in size, 
though the xylem remains approximately the same in amount, and 
parenchyma replaces the conducting tissue of the stele for a short distance. 
Sclerenchyma, however, soon takes the place of this thin-walled pith, and 
in correlation with the necessary increase in the assimilating tissue of the 
stem the cortical sclerenchyma decreases. 
1 Boodle (’ 04 ), p. 505. 
2 Lignier (’ 03 ), p. io 6. 
