Conducting Tissue- System in Bryophyta . 23 
come into lateral contact, separated by thick walls. The 
central hydrom-stereom mass is now surrounded by a com- 
plete investment, one or two cells thick, of starch-containing 
parenchyma with dark brown walls, continuous below with 
the pericycle and the starchy tissue internal to the leptom, 
and forming the characteristic starchy hydrom-sheath of the 
aerial stem. The furrows become shallower and gradually 
the hydrom-strand, which has by now lost the whole of its 
stereom, becomes cylindrical. At this level the files of large 
hydroids which occupy the centre of the stele are surrounded 
by scattered smaller ones, continuous with the small hydroids 
found in the pericycle of the rhizome, and these are mixed 
with moderately thick-walled starch-containing cells abutting 
on the dark brown hydrom-sheath. Higher up, these last cells 
are lost or absorbed in the hydrom-sheath, while the small 
hydroids increase in number and form the peripheral thin- 
walled hydrom-manfle of the stele of the aerial stem. The 
three leptom-strands are now completely outside the circum- 
ference of the cylindrical hydrom, from which they are separated 
by the fairly broad starchy sheath already described (Fig. 22). 
Externally they are bounded by smaller cells, which are often, 
however, not clearly distinguishable from those of the inner 
cortex. At this time each leptom-strand consists of two 
or three leptoids only, and it is only at a considerable higher 
level that the number is multiplied and lateral extension of 
each group begins, finally resulting in their fusion to form 
a complete peripheral leptom-mantle. Bastit figures this as 
taking place much lower down, before the peripheral ring 
of cortical stereom is complete (op. cit., Figs. 48, 49, p. 358), 
and he altogether misses the stage at which the still isodia- 
metric leptom-strands are entirely outside the cylindrical 
hydrom. 
Side by side with the lateral extension of the leptoids to 
form a complete leptom-mantle, the small peripheral hydroids 
increase in number and eventually form a complete wide zone 
rpund the larger central ones. At the same time obvious 
leaf-traces begin to appear in the cortex, in direct relation 
