80 
G. P. DARNELL-SMITH ON 
While Psilotum may be found in abundance upon exposed cliffs, particularly 
those facing the north, and which are hence exposed to the most prolonged sunshine, 
it is also to be found in deep gullies if the rocks are massive, and especially in the 
neighbourhood of small waterfalls. The water in the streams in these gullies fre- 
quently contains iron, probably as a sesquicarbonate. Where this water oozes 
through a rock it deposits a film of the hydrated oxide of iron. In time these films 
grow into layers of considerable thickness. Occasionally droplets of water become 
imprisoned in films of oxide of iron before they have had time to evaporate or dis- 
perse, and a spongy mass of globules of iron oxide, containing droplets of water in 
their interior, finally results. As the layers of oxide of iron increase in thickness 
the direction of the oozing water is slightly altered. In course of time earth and 
vegetable debris become mingled with the layers of iron oxide, and there results a 
more or less spongy or shaly mass of flakes of iron oxide, earth, and decayed 
vegetable matter adjacent to a constant supply of water. In such a porous mass 
that is always moist the rhizome of Psilotum will flourish most vigorously. 
If the rhizome of Psilotum penetrate to a spot saturated with water, it turns black 
and decays ; also, if it penetrate to a spot containing much clay, it seems unable to 
make progress — the rhizoids become clogged and the rhizome decays. The rhizome 
is never found at the ordinary ground-level, and seems unable to tolerate for any 
length of time stagnant water, soil that is not porous, or soil from which water 
■cannot drain away very freely. What it appears to require is an atmosphere that is 
■continuously damp rather than a soil that is wet. It flourishes most vigorously 
where the rhizoids can stand out freely from the rhizome, their tips only touching 
the surrounding medium, where the superstratum and substratum are continuously 
moist (though seldom, sometimes never, wetted by rain), and enclose between them 
a thin layer of decayed vegetable matter. 
The rhizome has been found traversing the tortuous tunnels made by white ants, 
the cavities in sponge-like rock formations, and in fissures in the rocky overarching 
roof of caverns. In this latter situation, if the lower plates of rock be split off by 
means of iron wedges, the aerial parts of the Psilotum plant are found to arise from 
a densely coiled and interlaced mass of rhizomes ; the mass may be half an inch in 
thickness. Where the fissure narrows in the region remote from the opening to the 
air, strands of rhizome densely covered with rhizoids, which stand out freely from the 
surface, are found arising from the central mass. Still further in, the rhizome, which 
possesses little penetrative power, flattens itself out and pursues its way to the 
remotest and finest parts of the fissure, where the lower slab of rock joins the main 
roof of the cavern. Psilotum also occurs in moist earth held loosely together by 
the roots of ferns, the felted mass being closely adherent to vertical or almost vertical 
masses of rock. In such cases the rhizome seeks a position between the rock and the 
adhering felted mass, and is characterised by the production of an enormous number 
of bulbils. 
