When an animal is struck by a blow, it does not respond at once. A
certain short interval elapses between the incidence of the blow, and
the beginning of the reply. This lost time is known as the latent
period. In the leg of a frog, the latent period according to Helmwoltz,
is about one-hundredth of a second. This latent period, however,
undergoes appropriate variation with changing external conditions. With
feeble stimulus, it has a definite value, which, with an excessive blow,
is much shortened. In the cold season, it is relatively long. Again,
when we are tired our perception time, as we may call it, may be greatly
prolonged. Every one of these observations is equally applicable to the
perception time of the plant. In Mimosa, in a vigorous condition, the
latent period is six one hundredth of a second, that is to say, only six
times its value in an energetic frog! Another curious thing is that a
stoutish tree will give its response in a slow and lordly fashion,
whereas a thin one attains the acme of its excitement in an incredibly
short time! Perhaps some of us can tell from our own experience whether
similar differences obtain amongst human kind or not? The plant's latent
period in our cold weather may be almost doubled. Ordinarily speaking it
takes _Mimosa_ about fifteen minutes to recover from a blow. If a second
blow be given, before the full recovery of its equanimity, then the
plant becomes fatigued, and its latent period is lengthened. When
over-fatigued, it may temporarily lose its power of perception
altogether, what this condition is like, my audience is only too likely
to realise, at the end of my long address!