Osmundacece. 
255 
sieve-tubes. It seems that these are best regarded as part of the 
phloem, the orientation of the elements being disturbed by the exit 
of the leaf-traces, a disturbance which also affects the pericyclic 
cells in the vicinity. Faull (’01) regarded them as part of the 
protophloem, but Seward and Ford (’03) show that in Todea they 
are developed after the large sieve-tubes lying nearer the xylem. 
The centre of the stele is occupied by a parenchymatous pith 
which in some species contains strands of sclerenchyma. Each 
strand of xylem consists of a solid mass of scalariform trachea: 
without internal parenchyma. The spiral protoxylems are situated 
in the concavity of the U-shaped strands, abutting on the 
parenchyma, while the strands of oval or circular cross-section are 
mesarch or destitute of protoxylem. 
Fig. 81. Todea barbara. Diagram of part of stem in transverse section, 
with two leaf-traces. The varying shapes of the xylem strands and the varying 
positions of the protoxylems are shown. From Seward and Ford. 
Thus the construction of the stele differs very markedly from 
that of any of the Ferns with which we have become familiar in 
the course of these lectures. 
The U-shaped strands are those which give off the leaf-traces. 
The outer part, connecting the limbs of the U, separates off as a 
slightly curved arc, and becomes surrounded by phloem, that which 
covers the convexity of the trace being continuous with the external 
phloem of the stele below, while that which lines the concavity is 
continuous with the external phloem of the stele above, in the same 
vertical line (Fig. 82). The limbs of the U remain behind in the stele, 
and pass on upwards to join similar strands coming from other nodes, 
eventually forming the leaf-traces of higher nodes. In Osmundaregalis 
according to Zenetti (Fig. 83) the left-hand strand (the cylinder being 
