596 Ford . — The Anatomy of Psilotum triquetrum . 
apical cell has never been observed dividing into two. On the other hand, 
radial sections of bifurcating branches have never shown an apical cell to be 
present at the summit of either branch, although we have seen that it 
is generally found in older ones. From this we might then conclude that 
a new apical cell arises later in each resulting branch in place of the original 
single one which was present below the level of forking. 
The Subterranean Stem. 
In accordance with his definitions of the external structure, Professor 
Bertrand has described in some detail the anatomy of the rhizome of 
Psilotum. The structure of a simple branch and of the various ‘ cladodes ’ 
is given, and more or less hard and fast rules laid down as to the appearance 
presented by the stele in each sub-division. As already stated in this 
paper, it has been impossible to distinguish satisfactorily these various 
underground structures from their external appearance ; and investigation 
of the anatomy, moreover, has not led in any way to their confirmation. 
The structure of the stele depends, for the most part, on the size and 
development of the branch in question, and not on the position it occupies 
in regard to the main mass of the rhizome. According to Bertrand, the 
typical structure of the stele in a simple branch consists of a diarch band of 
xylem, with spiral protoxylem-elements at the two extremities of the band. 
The present investigations have not confirmed this as being in any way 
constant or typical. The xylem-strand varies considerably in size and 
shape (Figs. 8, 9 and 10) ; in smaller branches two or three tracheids alone 
may be found, in stouter and more mature regions of the rhizome an 
irregular mass (Fig. 8) or a band-shaped strand 1 , or again a central group 
of xylem more or less circular in outline may be found. The tracheids are 
scalariform, with no parenchyma interposed, and the spiral protoxylem 
elements of Bertrand appear to be generally absent. Both in transverse 
sections and in series of longitudinal sections cut on a microtome no trace 
of spiral or annular elements are often found, the first elements to be 
differentiated at the apex being as a rule of the ordinary scalariform type 2 . 
These results therefore agree with the earlier researches of Russow 3 , who 
came to a similar conclusion in regard to the absence of typical spiral or 
annular elements in the protoxylem. 
A similar absence of typical protoxylem also occurs in the rhizomes of 
Loxsoma , and of some species of Schizaea and Anemia 4 . 
Figs. 10 and 11 represent the xylem of a branch of a rhizome below 
1 See also Boodle (’04), p. 500 , Fig. 46 . 
2 In some cases transverse sections have given one or two tracheids of smaller size than the rest, 
which might be taken for protoxylem. Further investigation of sections belonging to the same series 
have almost invariably shown that these elements merely represent the narrow pointed ends of 
scalariform tracheids of ordinary size. 
3 Russow (’ 75 ), p. 21. 4 Gwynne Vaughan (’01), p. 79; Boodle (’01), pp. 375, 381. 
