595 
Ford — The Anatomy of Psilotum triquetrum . 
branches the whole apex is surrounded by two or three foliage leaves, which 
serve as a protection, but it is easily recognized in longitudinal sections. 
In the stouter shoots, especially in those bearing the branches with fertile 
leaves and sporangia in all stages of development, the apex is much hidden. 
It is often difficult to distinguish from the leaves, each of which arises as an 
outgrowth of the apical tissues, and at an early stage may show considerable 
resemblance to the stem apex. The presence of an apical cell in the young 
leaf, however, cannot be stated as constant, though at times it appears to be 
present. Professor Bower 1 has also pointed out that it is often impossible 
to say at an early date whether a leaf will be a fertile one or not. 
The apical cell of the stem (Figs. 6 and 7) has a large nucleus and 
granular protoplasmic contents. It has been termed by Professor Bertrand 2 
a ‘ dermatogen ’ apical cell ; but the subsequent divisions in the cells which 
are cut off from it do not appear to bear out this statement, other layers 
being derived from these divisions besides the dermatogen itself. It has 
not, however, been possible to trace the connexion between the cells cut off 
from the apical cell and the developing stele. Each cell which is cut off 
from the apex appears to divide by a tangential wall into an inner and 
outer cell (Fig. 7), and the inner again divides so that three result, an outer, 
middle, and an inner. A vertical wall also arises at an early stage, but it 
has not been possible to determine satisfactorily any of the subsequent 
divisions. The outermost of the three cells resulting from the tangential 
divisions forms one of the cells of the dermatogen layer, which passes over 
into the epidermis of the stem. 
The stele in the stem branches ends in an apical meristem of 
undifferentiated elements with conspicuous nuclei. Between this meristem 
and the apex itself is a mass of tissue, the product of the subsequent 
divisions of the segments cut off from the apical cell ; the tissue is composed 
of irregular cells with comparatively large nuclei. 
In regard to the dichotomy of the aerial shoot it is difficult to make any 
definite statement. As Solms-Laubach 3 has pointed out, the behaviour of 
the apical cell, if present, is an important factor in such a question. In 
dichotomy the single apical cell may on the one hand divide medianly into 
two, or again it may disappear altogether before bifurcation occurs, two 
new apical cells being differentiated in the two resulting branches. The 
nature of the apparent dichotomy would be open to doubt if it was shown 
that the single original apical cell continued to exist as the apical cell of 
one of the branches, a new one being cut off laterally for the second branch. 
It has been impossible to say definitely what occurs in Psilotum. The 
confirmed this view, and is inclined to agree with Strasburger in regard to the absence of a distinct 
apical cell in the aerial stem, although he finds one to be present in the fruiting branches. 
1 Bower (’ 94 ), pp. 42 and 49 ; also Juranyi (’ 71 ), p. 177. 
2 Bertrand (’ 81 ), p. 441, et seq. 3 Solms-Laubach (’ 84 ), p. 17°. 
