1921] MEIER—ROOT TIP 123 
For that phase of the investigation relating to current intensity 
and time of exposure required to produce death, the seedlings were 
placed immediately after treatment in moist pine sawdust in glass 
battery jars. A stick the size of a lead pencil was pushed down 
next to the glass, the seedling placed in the opening thus made, and 
the sawdust carefully brought around the root. Figures on labels 
placed above each seedling served for identification when obser- 
vation was again made, usually 24 hours after treatment. From 
4-6 controls were placed in each jar of 12-15 treated seedlings. 
All seedlings were carefully selected with special reference to 
length and diameter, those not corresponding to type being rejected. 
Only roots of 15-20 mm. length were used and of diameter as nearly 
uniform as examination without actual measurement could select. 
No practical means suggesting itself of measuring the diameter of 
the roots before exposure to current, the approximate diameter of 
the roots used was found as follows. Free-hand sections were made 
of 125 roots (carefully selected as if for treatment with the current), 
3-4 mm. from the tip, the sections placed in a drop of water on a 
slide and diameter measured under the microscope with eyepiece 
micrometer. It was found that all fell within the following meas- 
urements: long diameter, 1.03-1.18 mm; short diameter, 0.84-0.96 — 
mm. It will be seen from these dimensions that the roots were 
somewhat flattened. The cross-sectional area of the roots varied 
then from approximately 0.7 sq. mm. too.9 sq.mm. The diameter 
is an important factor, since on it depends the density of any given 
current intensity per unit area. It should be understood that 
these ranges are the extremes, the majority exhibiting no such 
variation as the extremes might indicate. 
With the method described the actual current flowing through 
the tissue can be measured and read from the milliammeter, and 
calculated for the unit area. More or less just criticism is often 
made of the method of exposure to current of unicellular organisms 
in electrotactic experiments when the material to be examined is 
placed in liquid. The amount of current flowing through the 
organism depends therefore on whether or not the liquid surround- 
ing it is'a good conductor. If the liquid medium is a poor con- 
ductor, the current will pass in large measure through the organism; 
