362 Boodle . — ■ Anatomy of the Schizaeaceae . 
radial direction, and their function is probably connected with 
the aeration of the tissues of the stele. In Fig. 1, Plate XIX, 
the layer of suberized cells may be recognized by comparison 
with the high-power drawing, Fig. 2, where this tissue (s. L) 
is seen to lie between the sclerotic middle cortex, one layer 
of which is shown at the top of the drawing, and the en- 
dodermis (<?.). The endodermis is easy to distinguish; in 
young stems it shows the usual suberized bands in its cells, 
and in old stems all its cell-walls are suberized. Its cells 
become yellow or brownish. The pericycle is mostly from 
three to four cells thick. Its cells, at an early stage, show 
radial seriation with their radial walls corresponding with 
those of the endodermal cells (as seen to some extent in 
Fig- 3)- This arrangement is, however, upset by subsequent 
displacement (Fig. 2). The protophloem forms a ring, broken 
here and there, and consists of rather small sieve-tubes, often 
flattened in old stems, and easily distinguished from the 
succeeding band of metaphloem, which is composed of large 
sieve-tubes and phloem-parenchyma. The walls of the sieve- 
tubes are rather thick, and stain strongly with haematoxylene 
(Fig. 2 ,///.). In thick unstained transverse sections the sieve- 
tubes are seen to differ by their scanty contents from the 
phloem-parenchyma. 
In longitudinal section the sieve-tubes, which are much 
elongated elements with oblique end-walls, appear empty 
except for the presence of numerous granules on the walls. 
Usually, at any rate, no nuclei are present. This character 
readily distinguishes the sieve-tubes from the pericyclic cells, 
which have evident protoplasm and nuclei. The walls of the 
sieve-tubes stain blue with Schulze’s solution, often rather 
pale, but in some sections of the petiole they took an extremely 
deep-blue colour; in this case the conjunctive parenchyma- 
cells in the phloem and xylem were also deeply stained, 
but the pericyclic cells were scarcely coloured. The sieve- 
plates remain nearly unstained by Schulze’s solution, and 
the granules turn slightly yellow. The latter are often nearly 
restricted to the sieve-plates, and probably consist of proteid. 
