226 
Walter Stiles and Ingvar Jorgensen. 
sporogonium suggests that this genus must have come direct from 
a high type comparable to Marchantia , but now no longer existent. 
Notwithstanding the great reduction in the thallus of this genus 
due to its inhabiting dark places, it is still higher than Targionia in 
many respects—for example, the differentiation of the capsule 
into an operculum, a thickened part and an unthickened portion 
and its dehiscence by eight definite valves, the tuber-forming habit, 
and the composite male receptacle. 
For this reason in the preceding table (p. 209) giving the 
relationship of the Marchantiales, Cyathodinm is shown as descending 
directly from some Marchantia- like ancestor, while Targionia is the 
lowest link in a separate chain of types. The question of the origin 
of forms like Corsinia, Rehoulia , Plagiochasma and Riccia will be 
considered later. 
THE MEASUREMENT OF ELECTRICAL CONDUCTIVITY 
AS A METHOD OF INVESTIGATION IN 
PLANT PHYSIOLOGY. 
By Walter Stiles and Ingvar Jorgensen. 
[With Five Figures in the Text]. 
Introduction. 
I N this paper an attempt is made to present an account of the 
work which has been done in regard to the use of electrical 
conductivity phenomena in the solution of problems of plant 
physiology. It appears to be desirable to bring together in one 
paper references to the very scattered literature of the subject, 
more especially as considerable attention has been devoted to this 
question recently in various quarters. Further it would seem 
useful to call attention to the fact that the range of methods 
available for such work is much wider than is generally supposed ; we 
have therefore dealt with this aspect of the question, mentioning, as 
far as we are acquainted with them, the difficulties and advantages 
and disadvantages of the various methods, 
