Conducting Tissue-System in Bryophyta . 35 
in the leafy stem is clearly enough composed of the bases 
of the leptom-strands of the leaf-traces. Traced upwards, 
however, after the passing out of the last of the leaf-traces, 
it is continued upwards together with the altered cells of the 
hydrom-sheath, which no longer contain starch : consisting 
of a layer of thin-walled elongated cells, often with dense 
proteid contents, but in which no typical leptoids can be 
detected, it overlaps the central hydrom and comes into contact 
at a higher level with the base of the sporogonium, to which 
no doubt it supplies plastic substances for the formation 
of the young spore-capsule. Below the lowest leaf-trace the 
leptoids, as we have seen, decrease in number very consider- 
ably, and the remaining ones are concentrated in three strands 
which run down into the furrows of the hydrom-stereom 
cylinder of the rhizome. 
According to this view, the highly developed Polytricha- 
ceous stele is in the aerial stem essentially double in nature 
and phylogenetic origin, consisting (1) of a central primitive 
hydrom-cylinder, originally developed and still serving to 
supply the apical bud, sexual organs, and sporogonium with 
water, and (2) of a double peripheral mantle of hydrom and 
leptom separated by a starchy hydrom-sheath (amylom), all 
three layers composed of the joined bases of leaf-traces and 
designed between them to conduct water to, and formed 
material from, the leaves. 
The bearing of these considerations on the problem of the 
nature and origin of the primitive stele among the Pterido- 
phytes, as we find it for instance among the Sphenophyllineae 
and Lycopodineae, is a very interesting question which we 
cannot here discuss at length. We can only suggest the 
possibility of two alternative explanations of such a stele. 
Assuming first for simplicity’s sake the truth of the Bowerian 
theory, we may on the one hand suppose the primitive 
Pteridophyte descended from a form bearing a terminal fruit- 
body, rather of the nature of the sporogonium of an Anthoceros , 
and with a primitive hydrom-stele comparable with that of 
the Mosses, but supplying this fruit-body directly (since it 
