730 Gwynne- Vaughan. — Observations on the 
time it is never altogether absent from the adaxial side 
(Figs. 24, 25, 26). The protophloem is always quite distinct 
on the abaxial side, and it is nearly always to be found on the 
adaxial side as well. Indeed when the petiole contains a 
single strand only, the protophloem can often be followed 
all round its adaxial concavity. Whenever the xylem strand 
of the petiole is prolonged into a hook, the protophloem, if 
present, always passes straight across over the bay that lies 
between the hook and the main strand (Fig. 26). It appears, 
therefore, that the sieve-tubes situated within the bay itself 
must be regarded as belonging to the metaphloem. In most 
cases the sieve-tubes within the bay of the hook are quite 
normal (Figs. 25, 26), but in a good many Ferns they exhibit 
a special kind of structure. Their walls are unusually thick, 
and when unstained have a swollen pearly appearance. The 
additional thickening is all cellulose, and consists of two fairly 
distinct layers, the innermost of which is broader, softer, and 
less refractive than that next the middle lamella. They have 
rather more contents than the ordinary sieve-tubes, and the 
deeply staining granules characteristic of typical Fern sieve- 
tubes are rarely if ever to be found in them. For all that 
they do not seem to be essentially different from typical 
sieve-tubes, although it is doubtful whether they continue 
to function as such. One form of sieve-tube graduates insen- 
sibly into the other ; indeed, towards the base of the petiole 
the thick-walled sieve-tubes are nearly always replaced by the 
typical form. The thickened sieve-tubes may also occur 
all round the adaxial side of the leaf-trace ; especially near 
the protoxylems, and again on the abaxial side of the strand 
on the flanks of the xylem. They are especially well shown 
in the petioles of Dicksonia adiantoides, D. culcita , D. puncti- 
lofra, Davallia aculeata, D. platyphylla, and D, davallioides. 
These modified sieve-tubes occupy exactly the same position 
as the lignified ‘ phloem-fibres ’ in the leaf-traces of L ox soma 
Cunninghamii 1 and of certain species of Aneimia 2 , and 
1 Gwynne- Vaughan, 1. c., p. 83, Figs. 5 and 6. 
2 Eoodle, On the anatomy of the Schizaeaceae, 1 . c., p. 400. 
