I was speaking of the need of various delicate instruments—phytographs, as I shall call them—for the automatic record of the plant's responses. What was, ten years ago, a mere aspiration, has now after so many years of effort, become actual fact. It is unnecessary to tell here of many a fruitless and despairing attempt. Nor shall I trouble you with any account of intricate mechanism. I need only say that with the aid of different types of apparatus, it is now possible for all the responsive activities of the plant to be written down. For instance, we can make an instantaneous record of the growth and its variations, moment by moment. Scripts can be obtained of its spontaneous movement. And a recording arm will demorcate the line of life from that of death. The extreme delicacy of one of these instruments will be understood, when it is said that it measures and records a time-interval so short as one-thousandth part of a second!
It has been supposed that instruments for research of this delicacy and precision, were only possible of construction in the best scientific manufactories of Europe. It will therefore be regarded as interesting and encouraging to know that every one of these has been executed entirely in India, by Indian workmen and mechanicians.
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
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 observed. In man and other animals, there are tissues which beat, as we say, spontaneously. As long as life lasts, so long does the heart continue to pulsate. There is no effect without a cause. How then was it that these pulstations [became spontaneous? To this query, no fully satisfactory answer has been forthcoming. We find, however, that similar spontaneous movements are also observable in plant tissues, and by their investigation the secret of automatism in the animal may perhaps be unravelled.