Conducting Tissue- System in Bryophyta. 31 
With regard to the evolution of this complicated system 
of conducting tissues, we can follow in the completest way 
the different stages of its differentiation. 
The very earliest stages are not yet known among the 
Mosses themselves, but we find one type in the leaves of 
the leafy Liverwort Diplophyllum , and the other in the midrib 
of the thallus of the plant from Malahide at present known 
as Pallavicinia hihernica , var. (3, W ilsoniana. 
The first demand in a leafy plant vegetating in a damp 
atmosphere, and capable of absorbing water more or less 
over its whole surface, is probably for easy conduction of 
formed food-substances away from the leaves, rather than 
for the rapid supply of water to the leaves. The former is 
probably the function of the living elongated cells forming 
the midrib of the Diplophyllum leaf. The strands of pros- 
enchymatous lignified cells in the midribs of the thallus in 
P allavicinia, Symphyogyna , and Hymenophyton on the other 
hand are no doubt water-conducting, as we have shown in 
the case of P. Lyellii. Of these, the strands of P. hibernica 
(3, W ilsoniana are certainly the least differentiated, and in all 
probability the most primitive. The cells of the strand are 
more elongated than the cortical cells, but their walls, though 
slightly lignified, are scarcely thicker than those of the cortex. 
They are not entirely destitute of contents, though we have 
not been able to make out that any of them possess a nucleus. 
Taking all the facts into consideration, in the absence of 
experiment it is probable that they function as a water- 
conducting channel. From this type we pass to the very 
long and narrow, thick-walled, lignified, and richly pitted 
elements typical of the central strand in the three genera 
named. These may be regarded as comparatively highly 
differentiated hydroids. 
In the majority of the Mosses we have a similar stage of 
development in the central strand of the stem, except that 
here the hydroids are thin-walled and nearly always without 
pits. The leaves possess in their midribs elongated living 
cells, no doubt for the conduction of formed organic sub- 
