58 BOTANICAL GAZETTE [JULY 
It is seen from table II that oxidation by the extract of dis- 
eased bark is more rapid than by the extract of healthy bark, being 
greater at the end of five days than that caused by healthy extract 
at the end of seven days. : 
These two extracts were titrated immediately after extraction 
with n/zo NaOH, using phenolphthalein as an indicator. The 
difficulty of determining. the end point, due to darkening of the 
extract on addition of alkali, was avoided by using the test-plate 
method described by Miss ScHLEY (5). In this two phenolphthalein 
solutions are used, one slightly alkaline, the other slightly acid; the 
end point is reached when a drop of the solution being titrated just 
fails to decolorize a drop of the alkaline solution and just shows a 
faint pink tinge in a drop of the acid solution. By this method 
the following results were obtained, using 20 cc. of extract in 50 Cc. 
of distilled water: to neutralize 1 cc. of extract of healthy bark to 
phenolphthalein requires 0.86 cc. n/20 NaOH; to neutralize 1 cc. 
of extract of diseased bark to phenolphthalein requires 0.38 cc. 
n/20 NaOH. 
These titrations, of course, measure only the base-absorbing 
power of the extracts, not the H* concentration, for the extracts 
are probably mixtures of strong and weak acids and acid salts, all 
aving different degrees of ionization. But since the concentration 
of free H* ions is known to have a marked influence on many of the 
reactions taking place in living matter (HOBER, p. 176), it was 
important to determine just what this concentration is in the bark 
extracts. In the absence of facilities for doing this accurately by 
the gas-chain method, tests were made of the effect of diluted and 
undiluted extracts on the color changes of various indicators. The 
most clear-cut results were obtained with mauvein, which is known 
to give the following color changes: yellow at H* = 2X 107%; green at 
1073; green-blue at 107+; blue at 1075; violet at 107°. In repeated 
tests on extracts from different samples of diseased bark, mauvein 
was turned to a definite green, indicating that here H* is about 107°. 
Extract of healthy bark turned mauvein yellow, indicating that 
H*=2xX1075. It was found, however, that any given quantity of 
this extract had to be diluted to 2.4 times its original volume to 
give the same green as the extract of diseased bark; consequently 
