144 Knight and Priestley.— The Respiration of Plants 
evaporation, which in turn is caused by variations of initial temperature ; but 
no trials have been carried out to substantiate this. 
It will be seen that the curve of No. 113 is quite regular, and the same 
applies to the controls enumerated later, but Nos. 53 and 59 show irregu- 
larities, amounting to not more than about 2 or 3 per cent., at the fourth 
and fifth point respectively. In view of the later experiments, there is no 
doubt that these irregularities are due to slight changes of temperature, 
since similar discrepancies appeared in No. 113 and others before the tem- 
perature corrections were applied. Unfortunately, in the earlier experiments 
no record of temperature was obtained. 
Having then determined the course of the ‘ normal curve’ of respiration, 
it was obvious that there were two possible methods to adopt, viz. : 
1. To allow the seeds to remain under the conditions of the experiment 
for, say, three hours, till the variation of normal respiration was negligible, 
before making any determinations, and then to measure the carbon dioxide 
output for two successive periods, during the second of which the seeds 
were electrified. 
2. To make determinations of carbon dioxide output from the beginning 
and so ascertain the slope of the curve for the particular existing conditions, 
and then to apply the current and observe its effect upon the slope. 
The first method was employed in many of the earlier trials, and led 
to many misinterpretations of results. The chief objection to it is that, 
with so few points on the curve available, it is impossible to determine the 
normal slope, and, since the slope is liable to variations, this knowledge is 
essential. 
The second method has the disadvantage of requiring such a large 
number of determinations before a point of reasonable slope is reached. 
Consequently, a compromise was finally made, measurements of carbon 
dioxide being begun from one to two hours — a matter of convenience — 
after the introduction of the seeds, and generally continued for six or eight 
successive periods, each of half an hour’s duration, the current being applied 
during the fourth or fifth, whichever seemed more advisable. 
Some thirty experiments were carried out in this manner, the current 
densities employed varying from about io~ 6 amperes to more than io -4 
amperes, the intention being to imitate the current-conditions of the field 
trials (see Table II). In some of the early determinations it seemed as if 
the current had, indeed, produced an effect upon the evolution of carbon 
dioxide, but in the later ones, where temperature changes were accurately 
recorded, there was no deviation from the normal curve following the 
application of the current. The apparent effect referred to was evident in 
only a few cases, and its magnitude and even direction varied, and it is 
probable that it was due to unrecorded changes of temperature. 
When the current density approached the upper limit there was a slight 
