590 
Ford* — The Anatomy of Psilotum triquetrum. 
The Aerial Stem . 
The aerial stem forms externally the most noticeable part of a Psilotum 
plant. At its base, where it issues from the soil, it is smooth, brown, 
and circular in outline, but higher up it gradually becomes green in colour, 
and the surface is conspicuously ribbed. Branching is apparently dicho- 
tomous, the first bifurcation occurring as a rule near the base of the stem, 
and is repeated many times. Here and there trichotomy may occur, not 
only in the lowest portion of the stem, as Solms-Laubach 1 states, but also 
at the second, third, or fourth level of forking. Prantl 2 states that the 
branching of Psilotum cannot be regarded as dichotomous, for one of 
the branches arises in the axil of a leaf, whilst the other continues the 
growth and the phyllotaxis of the main shoot. In the case of P . flaccidum> 
however, in which the leaf-insertion is much clearer than in P. triquetrum , 
and is represented by the fraction J, Solms-Laubach 3 has shown that 
the position of the leaves is not altogether regular in regard to the branch- 
ing, whilst the phyllotaxis of the two branches points to dichotomy of 
the main shoot. As a rule, in P. fiaccidum a leaf is found below each 
bifurcation. The first few leaves belonging to the two resulting branches 
continue the distichous arrangement as if no forking had occurred ; thus, 
the first of these leaves is found on the outer side of the branch which 
is furthest away from the leaf below the bifurcation, whilst the second leaf 
is immediately above the latter and on the nearer branch. Higher up each 
branch assumes the usual distichous arrangement. It may, however, 
occasionally happen that the leaf usually found below the bifurcation is 
carried up, and is therefore situated on one of the two resulting branches. 
Judging from external appearances only, the branching would seem to 
be, as a rule, a regular dichotomy, although at times, especially towards 
the apex, branches of unequal size may be found. The question, however, 
will be referred to later in discussing the internal anatomy. 
Leaves are present as small, scale-like structures. The phyllotaxis 
cannot be represented by a fraction which is constant throughout the 
plant. As Solms-Laubach 4 has pointed out, in the smaller, three-angled 
branches near the apex the leaf-insertion is easily ascertained, and is clearly 
represented by the fraction J ; but in lower and stouter regions of the plant, 
owing to a certain extent to the twisting of the stem which often occurs, 
the leaf arrangement is much less clear and cannot be definitely or 
satisfactorily determined. 
The sporangia occur, as a rule, on the later-formed branches, but they 
may be found on older branches near the base of the stem. The sporangial 
1 Solms-Laubach (’84), p. 163. 2 Prantl (’72), p. 92. 
3 Solms-Laubach (’84), p. 165. PL XXIII, Figs. 10 and 1 1. 
i Solms-Laubach, loc. cit., p. 164. 
