Conducting Tissue-System in Bryophyta. 27 
pushes through the stelar leptom-mantle and its inner 
(pericyclic) cells just mentioned join the dark-walled starchy 
hydrom-sheath. At the same time the hydroids also rather 
suddenly increase in number, collecting together in a strand 
roughly circular in section (XI and XII), while the leptoids 
appear to go off right and left in two groups to form part 
of the stelar leptom-mantle (X and XI). The outer layer 
of dark brown-walled starchy parenchyma belonging to the 
trace is still seen immediately external to and on each side 
of the hydroid-strand (XII). The latter now pushes forwards 
and joins the peripheral thin-walled hydrom-mantle with 
which its elements are identical in every respect, the starchy 
sheath of the leaf-trace joining the starchy hydrom-sheath 
of the stele in the same way (traces XIII and XIV) 1 . 
The most important fact to notice in this series of pheno- 
mena is the formation, as it were, of the whole of the stem- 
stele with the exception of the central thick-walled hydrom- 
cylinder and the rudimentary pericycle, from the bases of 
leaf-traces. All the elements of the stelar leptom at a given 
level can be accounted for as belonging to about six leaf- 
traces (in Fig. 20 those numbered X to XV inclusive) com- 
pleting in the phyllotactic spiral about two circuits of the 
stele. The same may be said of the elements of the hydrom- 
sheath and of the thin-walled peripheral hydrom-mantle, 
though here, owing to the complete merging of the incoming 
1 Coesfeld (op. cit., p. 164) states that the large cells of the trace (our leptoids) 
enter the peripheral ring of the hydrom-cylinder together with the hydroids. We 
do not believe that this is the case. The cylindrical strand of thin-walled elements 
surrounded by a starchy sheath and representing the base of a trace (XI and XII) 
certainly differs from the row of isolated hydroids of the band-shaped upper portion 
of the trace ; the elements are much more numerous, are wider, and often have dis- 
organized contents. But they always differ from the leptoids of the upper part of 
the trace and of the leptom-mantle in having no nucleus. Together with the cells 
of the peripheral thin-walled mantle of the central part of the stele, with which 
they are continuous and identical in character, they represent, according to our 
theory (see third section), the last part of the hydrom-system to be evolved, the 
final connexion between the detached leaf-trace and the hydrom of the primitive 
stele. Their differentiation appears to be more rudimentary than that of the other 
hydroids. It may be that they help to conduct formed material under some circum- 
stances, but they clearly do not belong to the regular leptom-system. 
