Conducting Tissue-System in Bryophyta . 33 
is often quite unconnected spatially, though it clearly has 
a physiological connexion with the stele. It is only as a 
second thought, so to speak, that the establishment of a direct 
spatial connexion with the hydrom of the cylinder occurs 
to the trace, as being the simplest and most effective means 
of establishing a water-channel between the absorbing part 
of the stem and the leaf. In none of the higher plants, so 
far as we know, is anything similar found. However simple 
or complex the stele may be, the leaf-traces are always in 
direct connexion with it from the outset. This being so, it is 
difficult to believe that the stem-stele and leaf-bundle of the 
Mosses were evolved as parts of a single conducting system 
designed for a single purpose. The explanation that suggests 
itself to us is that the stem-stele originated in the first instance 
in order to supply the growing apex (cf. Strasburger, 
Practicum, xxi Pensum), the sexual organs, and particularly 
the developing sporogonium, with water ; while the leaf-bundles 
originated quite independently, at first probably in the form 
of a median strand of ‘ Leitparenchym 5 (as in the Liverwort 
Diplophyllum and many Mosses), to conduct the products of 
assimilation away from the leaf, hydroids being afterwards 
associated to facilitate the passage of water to, and possibly 
from, the leaf, as alternately the transpiring and absorbing 
organ. In those forms which adopted a purely terrestrial 
habit, rooted in soil and expanding their leafy shoots in air 
which was often relatively dry, the current of water up 
the stem became more constant and the leaves became more 
regularly and to a greater extent transpiring organs. A demand 
for easier conduction from stem to leaf was felt and was met 
by the continuation of the water-conducting part of the 
leaf-bundle into the stem, where it was expanded in various 
ways ( Mnium , Bryum , Splachmim ) in order to present a larger 
surface to the cortical tissue, so that the leaf-bundle could 
indirectly tap the water in the central strand to greater 
advantage. 
The habitats and habits of the plants belonging to these 
genera completely correspond with the existence of such 
D 
