With perfect instruments at our disposal, we may proceed to describe a few amongst the many phenomena which now stand revealed. But before this, it is necessary to deal briefly with the superstition that has led to the division of plants into sensitive and insensitive. By the electrical mode of investigation, it can be shown that not only Mimosa and the like, but all plants of all kinds are sensitive, and give definite replies to impinging stimuli. Ordinary plants, it is true, are unable to give any conspicuous mechanical indication of excitement. But this is not because of any insensitiveness, but because of equal and antagonistic reactions which neutralise each other. It is possible, however, by employing appropriate means, to show that even ordinary plants give mechanical replies to stimulus. THE DETERMINATION OF THE LATENT PERIOD 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! THE RELATION BETWEEN STIMULUS AND RESPONSE