576 Hume,— The Histology of the Sieve Tubes of Pteridium 
were remordanted in dilute iron alum and stained in water blue made up 
with aniline. 1 
The material of Marsilia quadrifolia and Lygodium dichotonmm was 
obtained from the University Botanic Garden, by the kindness of the Curator ; 
that of Pteridium aquilinum was collected in Gamlingay Wood. 
Distribution and General Characters of the Sieve 
Tubes. 
Both in Marsilia and in Pteridium the sieve tubes are characterized, 
as has already been stated, by their remarkable size (PL LV, Fig. 32). 
In Pteridium they occur as a complete ring surrounding the xylem in 
each of the vascular strands of the rhizome (PI. LV, Figs. 31 and 32). The 
vascular strands are flattened dorsiventrally, corresponding to the horizontal 
position of the rhizome, and it is on the dorsal and ventral sides of these 
strands that the phloem attains its greatest development. Laterally it is 
much thinner, the phloem elements being very small and scarcely distin- 
guishable from the protophloem. The ring of large sieve tubes is very 
regular, and is not usually more than one cell thick, so that each sieve tube 
abuts on another on either side of it, but centrifugally and centripetally it 
abu s upon phloem parenchyma, protophloem, or pericycle (PL LV, Figs. 31 
and 32). The sieve plates of the lateral walls, therefore, occur almost in- 
variably upon the radial walls, i. e. between two sieve tubes, and being in 
Pteridium of frequent occurrence, they form a very perfect, continuous con- 
ducting channel round each of the vascular strands. The importance of 
this will be realized when it is recalled that the roots occur all along the 
ventral side of the rhizome, while a single frond is given off laterally each 
year ; there is no aggregation of roots and leaves at a definite node, as will 
be described in Marsilia. 
The protophloem (PL LV, Fig. 29) occurs as an almost continuous ring 
of very small and, in the mature state, thick-walled elements, situated just 
outside the ring of large sieve tubes. Irregularly distributed, and abutting 
on the tangential walls of the sieve tubes, are the phloem parenchyma cells 
(PL LV, Fig. 31), which are elongated, with dense contents and a large 
elongated nucleus. Pits can be seen in the walls between sieve tubes and 
phloem parenchyma cells (PL LV, Figs. 30 and 32). The pericycle is 
generally one cell thick, and the cells are very obviously sister cells of the 
endodermis. Pits are particularly abundant on the tangential walls between 
pericyclic and endodermal cells. As regards the general characters of the 
mature sieve tubes, they are large, very thick-walled elements, with a thin 
parietal layer of protoplasm containing refringent granules. The lumen of 
the cell appears empty in cross section, but no doubt, in life, the contents 
1 Hill (’08). 
