Condiicting Tissue-System in Bryophyta . 19 
on the flanks is the layer of cells lining the furrow and con- 
tinuous with the pericycle. This layer is one or two cells 
thick, and corresponds in character with the pericyclic arcs, 
except that it is uninterrupted by hydroids, and so far as we 
have seen always contains starch, which the pericyclic arcs 
do not. To the outside of the bay the leptom is bounded 
by the inner narrower portion of the wedge-shaped radial 
strand, whose cells as already described are rather thin-walled 
and slightly lignified, somewhat broader than the leptoids 
and containing starch. 
The most striking feature in the arrangement of the fairly 
complicated tissue-systems of the Polytrichaceous rhizome is 
undoubtedly its practically complete identity with that 
obtaining in the root of a vascular plant. We have (1) a 
surface-layer bearing root-hairs, (2) a cortex bounded internally 
by a well-differentiated endodermis, (3) a pericycle, though 
incomplete and often interrupted, (4) a compact central cylinder 
in which most of the stereom is localized, and exhibiting (5) a 
definitely tri-radial arrangement of the leptom, the hydrom- 
elements being scattered among the stereids in the promon- 
tories between the leptom-furrows as well as in the centre of 
the cylinder. The existence of central tracheae mixed with 
mechanical elements is a fairly common feature in the roots 
of Angiosperms, especially among Monocotyledons. 
The only points in which the tissue-arrangement differs 
from the root-type are the incompleteness of the pericycle 
and endodermis. The former is not o’frly often interrupted 
by hydroids abutting on the endodermis, but, like the endo- 
dermis itself, is not continuous outside the leptom-strands. 
At these three points the periphery of the cylinder is, in fact, 
broken by the wedge-shaped masses of thick-walled cells 
which we have called the radial strands, leading out to the 
three hypodermal or scale-trace strands. In these last tissues 
we meet with the stem-nature of the rhizome, as shown by 
its bearing foliar organs. 
The tissues of the central cylinder, i. e. everything within 
the endodermis and radial strands, may be classified into 
