1922] ROBBINS—ROOT TIPS 65 
lost by contamination. The results of this experiment, so far as 
length increase is concerned, are presented graphically in fig. 3. 
Determinations of the a 
H-ion concentration of tty aa 
the solutions used in this 2-1) 5 
experiment were made by oe 
GILLESPIE’s} method at 
the beginning and at the 
end of the first, second, + 
and third periods. The 
modified Pfeffer’s solution -&+ 
at the beginning of each am 
period had a Py of 4.5. tottharhs 
The addition of the quan- (>the 
tity of peptone used made : Dui eee aes 
. IG. 3.—Growtnh in length of root tips of corn in 
“a mes es dark Ab. four periods; anaes aie modi- 
? H oo aes | -bescoerhe ~— — 2 per cent dextrose 
The autolized yeast had (check), 
a similar though less Pp yr o #0 pom pon ore stl 
marked effect, raising the aooidinie th wi period. j 
P, to 5.2. The growth 
of the roots made the check solution more alkaline (table II), raising 
the P, in the first period to 6.3~-6.8, in the second to 5.04, and barely 
affecting it in the third period. Both the yeast and the peptone 
exerted a buffer action, and little change in the reaction was 
produced in these solutions by the growth of the roots. It was 
noted that in the Pfeffer’s solution containing glucose the change in 
reaction was roughly proportional to the amount of growth which 
the roots made. Root tips which grew but little raised the P,, from 
4-5 to 5.1 in the first period; those which made the most growth 
raised it as high as 6.8. Higher values than this have been obtained. 
In one case root tips of Silver Mine corn grown for three months in 
the modified Pfeffer’s solution plus 1 per cent cane sugar (inverted 
in sterilizing) changed the P,, to as high as 8.4. 
Experiment 1o.—In this experiment the methods followed were 
as in the first experiment, but two concentrations of peptone were 
3GiLtespie, L. J., Colorimetric determination of H-ion concentration without 
buffer mixtures. Jour. Amer. Chem. Soc. 42:742-748. 1920. 
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