Boodle. — Anatomy of the Schizaeaceae . 36 3 
Part of a small sieve-tube from the rhizome of L . dichotomum 
is shown in Fig . 7. In the largest sieve-tubes the sieve-plates 
on the vertical walls are often much more crowded than in the 
specimen figured ; the thick parts of the wall, in consequence, 
have a reticulate arrangement. Sieve-plates in section and 
surface view are easily seen in unstained longitudinal sections. 
In structure the sieve-tubes of Lygodium agree with those of 
other Ferns, but callus, as will be described below, appears to 
be absent. 
A layer of conjunctive parenchyma ( 1-2 cells thick) separates 
the phloem from the xylem. The xylem occupies the centre 
of the stele, and forms a solid rounded mass about f mm. to 
2| mm. in diameter (Fig. 1). It consists of tracheides and 
parenchyma, the latter occurring chiefly as tortuous chains 
of cells, which are mostly uniseriate (Fig. 2). Short rows of 
cells and isolated cells (as seen in transverse section) also 
occur, but it is probable that the parenchyma really forms 
a connected system. The smallest tracheides, which are also 
the first to differentiate, and must be described as protoxylem, 
are scattered round the periphery of the xylem, and are not 
arranged in definite groups ; see Fig. 2, and also the young 
stem Fig. 3, where the protoxylem-elements are the only 
tracheides differentiated. Especially in large steles in this 
species the protoxylem-elements may be much more numerous 
on the lower than on the upper side of the xylem, and may 
differentiate earlier on the lower side. The first-formed 
tracheides differ from typical protoxylem in not being annular 
or spiral. They are finely scalariform. This character no 
doubt implies that the formation of the protoxylem is sub- 
sequent to the elongation of the tissues. The tracheides of 
the remainder of the xylem are scalariform, and their order 
of differentiation is not regularly centripetal, but a number of 
single tracheides scattered throughout the xylem become fully 
developed, while the intervening ones are still thin-walled. 
The latter become subsequently thickened and the mature 
structure is attained. The intermediate stage of development 
with internal scattered tracheides is described by Russow ( 72 , 
