186 MR. P. GROOM—CONTRIBUTIONS TO THE 
be most rapidly draining the roots of any plastic products, z. e. 
it was flowering (cf. the disappearance of starch in rhizome of 
Corallorhiza only when the flowers are forming). It may be 
suggested that the mycelial masses also have been absorbed from 
all parts of the roots save the youngest. 
(5) Endodermis (Pl. V. fig. 6).—In transverse section this 
single layer has an exceedingly characteristic appearance; it 
belongs to the C-type, with the lateral and inner walls thick- 
ened. The cells composing this layer are of unequal size. 
Outside each bundle of phloém there may (still in transverse 
section) be either one or two large endodermal cells with 
very thick suberized C-walls. On each side, going towards 
the xylem radii, the endodermal cells dwindle in size and their 
walls become thinner. Outside each group of xylem stand one 
or two passage-cells with feebly suberized walls. Longitudinal 
sections reveal the fact that the endodermal cells are elongated 
in the direction of the axis, and that the passage-cells form, not 
isolated spots but continuous lines or bands, outside the bundles 
of xylem. 
The stele commences with a 
(6) Pericycle, a single layer of short parenchymatous cells. 
Outside the bundles of phloém these have thick, lignified, pitted 
walls. Outside the bundles of xylem some of the cells retain 
their thin, unmodified, cellulose-walls. 
The central cylinder is pentarch, and the whole of the con- 
juncüve tissue is constituted of sclerenchymatous fibres. Thus 
it comes that the five feeble bundles of phloém are completely 
encased in cells with thick lignified walls. 
(7) Each bundle of phloëm consists of a few narrow sieve-tubes, 
fine companion cells with strap-like nuclei, and elongated paren- 
ehymatous cells with oval nuclei. 
(8) Each bundle of zylem shows an increase in the calibre of 
the water-conducting constituents from without inwards. These 
form a radial row, and appear to be tracheo-tracheides. 
(9) The bundles of xylem do not meet in the centre, so that 
there is a sclerenchymatous pith. 
The intercellular system is not well developed in the cortex, 
and it almost completely disappears within the stele. 
At the base of the root the inner layers of the cortex are 
converted into mechanical tissue composed of cells with thick 
lignified walls. 
