ation. But in the second case, the strain is not
completely removed after any single stimulation of the series. That
recovery is partial is seen by the gradual shifting of the base line
upwards. In the former case the base line is horizontal and represents a
condition of complete equilibrium. Now, however, the base line, or line
of modified equilibrium, is tilted upwards. Thus even in this case if we
measure the heights of successive responses from the line of absolute
equilibrium, they will be found to increase with increasing stimulus.
Ordinarily, however, we make no allowance for the shifting of the base
line, measuring response rather from the place of its previous recovery,
or from the point of modified equilibrium. Judged in this way, the
responses undergo an apparent diminution.

FOOTNOTES:

[13] See 'On Electric Touch,' _Proc. Roy. Soc._ Aug. 1900.

CHAPTER VIII

PLANT RESPONSE--ON THE INFLUENCE OF TEMPERATURE

Effect of very low temperature--Influence of high
temperature--Determination of death-point--Increased response as
after-effect of temperature variation--Death of plant and abolition
of response by the action of steam.

For every plant there is a range of temperature most favourable to its
vital activity. Above this optimum, the vital activity diminishes, till
a maximum is reached, when it ceases altogether, and if this point be
maintained for a long time the plant is apt to be killed. Similarly, the
vital activity is diminished if the temperature be lowered below the
optimum, and again, at a minimum point it ceases, while below this
minimum the plant may be killed. We may regard these maximum and minimum
temperatures as the death-points. Some plants can resist these extremes
better than others. Length of exposure, it should however be remembered,
is also a determining factor in the question as to whether or not the
plant shall survive unfavourable conditions of temperature. Thus we have
hardy plants, and plants that are affected by excessive variations of
temperature. Within the characteristic power of the species, there may
be, again, a certain amount of individual difference.

These facts being known, I was anxious to determine whether the
undoubted changes induced by temperature in the vital activity of plants
would affect electrical response.

#Effect of very low temperature.#--As regards the influence of very low
temperature, I had opportunities of studying the question