330 BOTANICAL GAZETTE [APRIL 
while all of the chlorides of bivalent kations are able to do so to 
a marked degree. Various tempting hypotheses are suggested by 
these striking facts. None of them can be worked out at present 
in a manner which is free from objection, and the writer deems 
it advisable to defer discussion of them until further investiga- 
tions have been made. 
Summary 
There is a remarkable difference between monovalent and 
bivalent kations in their effects on permeability. 
While none of the monovalent kations (except H) are able 
to decrease permeability, all the bivalent kations so far investi- 
gated (Mg, Ca, Ba, Sr, Mn, Co, Fe, Ni, Zn, Cd, Sn) are able to do 
so to a marked degree. 
HARVARD UNIVERSITY 
