598 Ford . — The Anatomy of Psilohim triquetrum. 
The Apex of the Subterranean Stem. 
The apex of an underground branch of Psilotum is terminated, as 
in the aerial branches, by a three-sided apical cell. The walls, too, arising 
in the segments cut off from this apex appear to be similar to those in 
the aerial part of the plant ; and again no connexion has been traced 
between the apical meristem, in which the stelar tissue ends, and the apical 
cell itself. 
The first xylem tissue to be differentiated and lignified appears to 
be often a single, ordinary, scalariform tracheid (Fig. 12); but in other 
cases two tracheids may appear at approximately the same time, lying 
either side by side, or separated from each other by one or more 
parenchymatous cells. 
The presence of one of Professor Bertrand’s ‘ cladodes ’ is said to 
be marked by more than one * centre of growth ’ being found at the 
apex, each of which has a distinct apical cell. The apical cells in question 
may lie in the same or different planes. It is certainly true that towards 
the apex of a branch the lateral buds may often be present in large 
numbers, and these may either grow out directly into ordinary branches* 
or again may remain dormant, becoming more widely separated from 
each other by the subsequent increase in length of the branch. Transverse 
and longitudinal sections cut on a microtome do not, however, seem to 
show at any time that more than one apical cell is present at the actual 
apex itself, although a somewhat lateral cell may at times be found 
belonging to a dormant bud. It is often impossible to detect an apical 
cell or cells in a branch which is about to bifurcate, although after forking 
these may make their appearance in the two resulting branches. This 
may point, as in the case of the aerial stem, to the fact that the cells in 
question are developed secondarily from the apical meristem of their 
respective branches. In no case has an apical cell been observed which 
is dividing medianly into two, this fact agreeing with the investigations 
of Solms-Laubach 1 . 
The Intermediate Region . 
This region includes the base of any aerial stem, from the level at 
which the external appearance gradually changes from the green ribbed 
surface to the smooth brown exterior, down to the point of insertion of 
this vertical stem on the rhizome. 
At the extreme base the stele of the stem possesses more or less 
the structure found in an ordinary rhizome branch, viz. a central strand 
of xylem of varying size and shape, with no clearly defined protoxylem, 
and surrounded by phloem-tissue. The walls of the two or three cortical 
1 Solms-Laubach (’ 84 ), p. 171. 
