732 Boodle . — Anatomy of the Gleicheniaceae . 
occur rather frequently. A root-stele passes through the 
cortex of the stem, and enters a short spine-like projection, 
which is the free part of the root. As the external tissues 
at its tip become sclerotic, it appears to be incapable of 
further growth. The stele in this spine and the root-trace 
in the cortex of the rhizome were found to be tetrarch in 
G. dichotoma and G. dicarpa , but differed from the normal 
roots of these species in having a tendency to form a pith. 
Thus in an arrested root of G. dicarpa the four xylem-groups 
did not meet at the centre but abutted on a small pith. 
A section through the trace of this root in the cortex of the 
rhizome showed a similar structure, except that one of 
the pith-cells near the centre was replaced by a tracheide, 
while two or three others appeared to have begun to thicken 
their walls and then to have undergone partial mucilaginous 
degeneration. Probably, when the arresting cause affected 
the organ, no further tracheides were differentiated, and con- 
sequently in the free part of the root the elements in the 
central part of the stele, which normally would have developed 
as tracheides, remained parenchymatous to form a pith. In 
an arrested root of G. dichotoma most of the central tracheides 
had been formed, both in the root-trace and in the spine. 
These points are mentioned partly as an analogy illustrating 
what may perhaps have taken place in the stem of certain 
Ferns, and led to the production of a pith. In this con- 
nexion it is as well to refer to the occasional occurrence of 
half-thickened and partly collapsed or even scarcely thickened 
elements, evidently representing tracheides. They were found 
sometimes as a group of two or three in the xylem of other- 
wise typical roots, and also near the centre of the xylem 
in the rhizome in specimens of three or four species. They 
sometimes also occur in petiolar bundles, and are apparently 
quite unconnected with any external mechanical injury. 
They may perhaps be due to the plant having grown, 
whether in the open or in cultivation, under conditions differ- 
ing from those of the normal habitat of the species. 
In species with a lobed xylem-mass in the rhizome, the 
