594 Ford — The Anatomy of P silo turn triquetrum. 
refractive spherules described by Poirault 1 as occurring in the sieve-tubes 
of many ferns. No nuclei could be found In these tubes, and no trace 
of callus could be demonstrated by treatment with coralline soda or with 
a watery solution of aniline blue. The phloem, or phloem tissue, is not 
stained blue by iodine as in the case of Lycopodium. Owing to the fact 
that no definite pores or sieve-plates have been observed in this tissue, it is 
of course open to question whether in Psilotum the term sieve-tube can be 
applied. It has been likewise impossible to locate any definite tissue 
corresponding to the protophloem, although Russow 2 in 1872 figured and 
described such elements as forming regular groups alternating with the 
protoxylem masses. Later, however, he modified his earlier view, stating 
that the protophloem (defining this as the first of the sieve tissue to be 
differentiated) consisted of numerous groups, each of which was composed 
of two or three elements lying at the periphery of the vascular strand. 
Bifurcation of the stem is very simple (Fig. 1). The stele widens 
horizontally and new protoxylem-groups make their appearance (Fig. i, p'x). 
The central mass of fibres with the surrounding xylem splits into two 
approximately equal halves, and new tracheids arise so that each of the two 
resulting groups of fibres are surrounded as before by xylem, though for 
a short time they touch the phloem tissue directly. The two strands 
become further apart, the endodermis then breaks and closes up round 
each. This process Is repeated at each level of bifurcation. 
Unlike Tmesiptevis , in which each leaf receives a small vascular strand 
from the stem, the leaves of Psilotum have no vascular bundle. 
In regard to the region in which the sporangia occur, each sporangio- 
phore receives a small vascular strand from the main stem. In stouter 
branches this strand is much the smaller, but towards the apex, where the 
stem branches are slighter, it is frequently equal in size to the main stele 
left in the stem. 
The Leaf. 
The leaf of Psilotum has a very simple structure (Fig. 5), the outermost 
parenchymatous zone and the epidermis of the stem alone being continuous 
with the tissues of the leaf. Externally the leaf is covered by a typical 
epidermis with cuticularised outer walls, but no stomata are present. The 
centre of the leaf is occupied by parenchymatous tissue ; near the apex this 
consists of elongated, nucleated cells, which, lower down, gradually pass 
into the parenchyma of the stem with its characteristic intercellular 
spaces. 
The Apex of the Aerial Stem. 
The apex of an aerial branch of Psilotum ends in a conical prominence, 
at the summit of which a three-sided apical cell 3 is found. In the smaller 
1 Poirault (’93), p. 139. 2 Russow (’72), Taf. XI, Fig. 30; also (’75), pp. 20 and 40. 
3 Strasburger ascribes an apical cell only to the underground stem. Solms-Laubach has 
