102 Brown. — Studies in the Physiology of Parasitism . VIII. 
For the determination of conductivities, the method described by 
Blackman and Paine 1 was employed. The cell consisted of a fine pipette 
with small bulb blown near the pointed end, into which the platinum 
leads were fused. The bulb was of such a size that it was completely filled 
by the drop. By means of a rubber cap with screw adjustment on the 
distal end of the pipette, the position of the drops in the cell could be 
accurately controlled. The use of a constant temperature bath was 
dispensed with and the readings made at laboratory temperature. In all 
cases* the temperature was noted so that correction of results could be 
effected. In view of the magnitude of the differences observed, the error 
due to slight variations in temperature could be ignored. 
To determine the capacity of the drops to stimulate spore germination, 
they were placed on clean glass slides, and a drop of a suspension of spores 
in water was added to each. Drops of water which had lain for twenty-four 
hours on glass slides or which came directly from the stock of distilled 
water were similarly treated and served as controls. After a certain time 
the state of the drops as regards germination of the spores was determined. 
In this connexion a mere count of the percentage of germinated spores is 
of doubtful value, as it fails to distinguish between a case where a certain 
percentage of spores germinates feebly and one in which a similar percentage 
germinates vigorously. The figure obtained in this way may thus give no 
true representation of the germinative picture displayed. A more accurate 
method was found in the measurement, by means of a micrometer eye-piece, 
of the total length of germ-tube of a number of spores chosen at random. In 
this way the ‘ average germ-tube length ’ could be determined. In view of 
the amount of variation which obtains between individual spores, a large 
number of counts and measurements (25, 50, or 100, according to circum- 
stances) was made in each case. This method is also incomplete, inasmuch 
as it fails to represent the difference between the thin feeble germ-tubes 
which are formed in water or very dilute nutrient and the stout vigorous ones 
formed in more concentrated nutrients, but it seemed to be the best available. 
In general, each drop was examined according to both methods — that 
is, it was removed from the plant surface by means of the pipette and 
its conductivity determined. It was then placed on a glass slide, the 
suspension of spores added, and the amount of germination ensuing in 
a given time measured. In this way a duplicate series of results was 
obtained. The electrical method gives only a measure of the amount of 
. dissociated electrolytes which have diffused out of the plant ; for the 
purpose of determining the amount of nutrient material appearing in the 
drops, the spore-germination method obviously is the direct one. The 
degree of parallelism exhibited by the two series of results will be illustrated 
subsequently. 
1 Ann. Bot., vol. xxxii, 1918, p. 69. 
