414 BOTANICAL GAZETTE [DECEMBER 
of variation was found. In but one instance did a salt give a 
boundary concentration greater than 0.04 gm. mol. per liter 
(sodium hippurate, 0.08 gm. mol. per. liter), and in a number of 
cases the alge did not survive until a solution containing but 1 
gm. mol. in 200 liters was reached, viz., sodium cinnamate and 
sodium protocatechuate. In no case does it appear probable 
that osmotic action plays any noticeable réle in bringing about 
the death of the plants; in all cases, therefore, practically toxic 
action only can be invoked as the cause of death. Assuming 
roughly dissociation in both of these salts to be approximately 
15 per cent., the osmotic value of these compounds would be 
1.15, that of cane sugar taken as unity. On the assumption of 
purely osmotic action, the theoretical boundary concentration 
would lie for each of these salts at about 0.65 gm. mol. per liter. 
Against these roughly calculated values may be contrasted the 
experimental boundary concentration, 0.005 gm. mol. per liter. 
The sodium compound most easily tolerated, sodium hippurate, 
is seen to act chiefly through its toxic properties, the cinnamate 
and protocatechuate practically entirely so. 
It is interesting in this connection to note that boric acid, 
the much used antiseptic, shows for Spirogyra less toxicity than 
any of the above mentioned sodium salts. 
Substances 
: ee ear 7 0.98 og 0.46 0.50 0.67. 9.75 
Cane sugar. 5. eos ‘ : p : ae 
Glycerine . . . : : , : Be } 
NaCl. ; : ; 
Bag ae é p t 
Sodium hippurate - : : / 
Sodium cinnamate | ¢ —:__° : : : a 
In closing this review of the experimental details, a diagram 
is given which presents in a somewhat more graphic form the 
