g. 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!

THE RELATION BETWEEN STIMULUS AND RESPONSE

According to varying circumstances, the same blow will evoke responses
of different amplitudes. Early in the morning, after the prolonged
inactivity of a cold night, we find the plant inclined to be lethargic,
and its first answers correspondingly small. But as blow after blow is
delivered, this lethargy passes off, and the replies become stronger and
stronger. A good way to remove this lethargy quickly, is to give the
plant a warm bath. In the heat of the midday, this state of things is
reversed. That is to say, after giving vigorous replies the plant
becomes fatigued, and its responses grow smaller. This fatigue passes
off, however, on allowing it a period of rest. On increasing the
intensity of the impinging stimulus, the response also increases. But a
limit is attained, beyond which response can no longer be enhanced.
Again, just as the pain of a blow persists longer with ourselves, in
winter than in summer, so the same holds good of the reaction of the
plant also. For instance, in summer it takes _Mimosa_ ten to fifteen
minutes to recover from a blow, whereas in winter the same thing would
take over half an hour. In all this, you will recognise the similarity
between human response and that of the plant.

SPONTANEOUS PULSATION

In certain tissues, a very curious phenomenon is