There are other instances of diurnal movement, far more numerous, which cannot be explained from considerations given above. It has therefore been suggested that the "Day and night positions may arise by the combined action of geotropism and heliotropism. Thus Vochting (1888) observed in the case of _Malva verticillatta_, that the leaves, when illuminated from below, turned their laminae downwards during the day, but during the night became erect geotropically. The sleep movements in leaves and flowers, referred to above, cannot however be explained by a.s.suming such a combination of heliotropism and geotropism."[40]

[40] For further information on the subject of Nyct.i.tropism, _cf._-- Pfeffer--_Ibid_, Vol. II (1903), p. 112; Jost--_Ibid_, pp. 500, 507; Vines--Physiology of Plants (1886), pp. 406, 543.

I commenced my investigation on nyct.i.tropism five years ago, after having perfected an apparatus for continuous record of the movements of plants throughout day and night. A contrivance, described further on, has been devised for obtaining a record of diurnal variation of temperature. I have also succeeded recently, in perfecting a device for automatic record of variation of intensity of light. It has thus been possible not only to obtain a continuous record of the diurnal movement of the plant, but also obtain simultaneous record of those changes in the environment which might have an influence on the daily movement. I have in this way collected several hundred autographs of different plants throughout all seasons of the year. The records thus obtained were extremely diverse, and it was at first impossible to discover any fundamental reaction which would explain the phenomenon. While in this perplexity my attention was directed two years ago to the extraordinary performances of the "Praying Palm" of Faridpore, in which the geotropic curvature of the tree underwent an accentuation during fall of temperature, and a diminution during rise of temperature.

The discovery of this new phenomenon led me to the inquiry whether Thermo-geotropic reaction, as I may call it, was exerted only on Palm trees, or whether it was a phenomenon of universal occurrence. I therefore extended my investigation on various geotropically curved proc.u.mbent stems of _Ipoemia_, _Basella_, and of _Tropaeolum majus_.

Here also I found that diurnal variation of temperature induced a periodic movement exactly similar to that in Palm trees.

I next wished to find whether the Thermo-geotropic reaction observed in stems was also exhibited by lateral organs such as leaves, which being spread out in a horizontal direction are subjected to the stimulus of gravity. I found that in a large number of typical cases, a periodic movement took place which was exactly similar to that given by rigid trees and trailing stems. A standard curve was thus obtained which was found to be characteristic not only of trees and herbs, but also of leaves. The stem and leaves _fell_ continuously with the rise of temperature, from the minimum at about 6 in the morning to the maximum at about 2 p.m. They erected themselves with falling temperature from 2 p.m. to 6 a.m. next morning.

In the diurnal record of _Mimosa_ I met, however, with an unaccountable deviation from the standard curve, for which I could not for a long time find an adequate explanation. Subsequent investigations showed that the deviation was due to the introduction of additional factors of variation, namely of immediate and after-effects of light.

COMPLEXITY OF THE PROBLEM.

I have already referred to the great difficulty of explanation of nyct.i.tropism from the fact that the diurnal movements may be brought about by different agencies independent of each other. It is, moreover, not easy to discriminate the effect of one agency from that of the other.

The combined effects of different factors will evidently be very numerous. This will be understood from consideration of the number of possible combinations with only two variables, geotropism and phototropism. The effect of geotropism may be strong _G_, or feeble, _g_. Similarly we may have strong effect of light _L_, or feeble effect of light _l_. Light may exert positive phototropic action +_L_ or negative action -_L_. Thus from two variables we obtain the following eight combinations:

_G_ + _L_; _G_ - _L_; _G_ + _l_; _G_ - _l_; _g_ + _L_; _g_ - _L_; _g_ + _l_; _g_ - _l_.

The number of possible variables are, however, far more numerous as will be seen from the following:

_Geotropism._--The effect of geotropic stimulus on horizontally placed organs is one of erection. But this stimulus, which is constant, cannot by itself give rise to periodic movements. It has however been shown that variation of temperature has a modifying influence on geotropic curvature (p. 519).

_Phototropism._--The action of unilateral light is to induce a tropic curvature, which in some cases is positive, in others negative (p. 386).

In addition to these effects induced during the incidence of light, we have to take account of the after-effects on the cessation of light.

_After-effects of light._--I find two very different effects, depending on the intensity and duration of previous illumination. Of these the most important is the phenomenon of 'overshooting' which occurs on the cessation of light of long duration. This particular reaction, to be fully described, will be found to offer an explanation of certain anomalous effects in diurnal movement.

_Periodic variation of turgor._--I have shown (p. 39) that artificial enhancement of turgor in the plant induces an erectile movement of the leaf of _Mimosa_, diminution of turgor inducing the opposite movement of fall. Kraus and Millardet have shown that a diurnal variation of tension takes place in the shoot of all plants, which is presumably indicative of variation of turgor. This variation of turgor in the shoot must have some effect on the lateral leaves. But the leaves are subjected to conditions which are absent in the stem. The erect stem is, for example, free from geotropic action, whereas the lateral leaf is subject to it.

The effect of turgor variation in the shoot on the movement of leaves may be, and often is, overpowered by the predominant geotropic action. I shall, later on, refer to this question in greater detail.

[Ill.u.s.tration: FIG. 188.--Arrest of pulsatory movement of leaflet of _Desmodium gyrans_ by light from above and gradual restoration on cessation of light. Up-movement represented by up-curve.]

_Autonomous movements: Experiment 202._--The lateral organ, say the leaf or leaflet, may have an autonomous movement of its own. In some, the autonomous movement may be relatively quick; the complete pulsation in _Desmodium gyrans_ may be as short as a minute or so. I find that this autonomous movement becomes modified or even arrested by the paratonic effect of light. This is seen in figure 188, where light applied from above is seen to arrest the pulsation; the normal activity is, however, restored on the stoppage of light.

[Ill.u.s.tration: FIG. 189.--Effect of unilateral light on hyponastic movement of the cotyledon of _Pepo_. Application of light indicated by arrows; light acting from below r.e.t.a.r.ds, acting from above accelerates the movement. The last part of the curve in each shows recovery on the stoppage of light.]

_Epinasty and Hyponasty: Experiment 203._--There are other autonomous movements which are relatively slow. Even in an erect stem there may be a to and fro oscillation. In such a case the effect of an external stimulus, say of light, is one of algebraical summation. The following is the summary of results of unilateral action of light on the nutating hypocotyl of a pea seedling:

+--------------------------------------------------------------------+

Natural movement.

Effect of light applied on the right side.

+------------------------+-------------------------------------------+

Movement to the right

Acceleration of existing movement.

Movement to the left

r.e.t.a.r.dation, arrest or reversal of natural

movement.

+--------------------------------------------------------------------+

Figure 189 exhibits the effect of light applied alternately above or below the cotyledon of _Cucurbita Pepo_. On account of the more rapid growth of the lower side, the cotyledon was exhibiting a hyponastic up-movement. Application of light from above enhanced the existing rate of movement, whereas light acting from below r.e.t.a.r.ded the movement. Here we have instances of photo-hyponastic modification of natural movement.

Similarly epinastic organs will, normally speaking, have their natural down movement r.e.t.a.r.ded by light from above, and accelerated by light from below. If the periodicity of the autonomous movements coincides with the periodicity of the external stimulus, the resulting movement will be determined by algebraical summation; it will be very p.r.o.nounced when the two effects are concordant. If the two periodicities do not agree, the interference effects will become extremely complicated.

_Positive thermonasty._--Rise of temperature inducing differential growth brings about the _closure_ of the flower. Fall of temperature on the other hand induces a movement of opening. Example of this has already been given in the responsive movement of _Nymphaea_.

_Negative thermonasty._--The opposite type of movement is exhibited by _Crocus_ and _Tulip_. Pfeffer has shown that a rise of temperature induces in these flowers, a quicker rate of growth of the inner side of the perianth. Rise of temperature thus induces a movement of opening, and a fall of temperature brings about the opposite movement of closure.

I shall presently describe the effects of both positive and negative thermonasty, in diurnal movements of flowers.

_Thermo-geotropism._--I have already described the accentuation of geotropic curvature during the fall, and a flattening of curvature during the rise of temperature (p. 519). The influence of this factor on diurnal movement will presently be treated in fuller detail.

There are thus more than ten variables, and the resulting effect due to their combinations will exceed a thousand. This will explain why attempts at explanation of the phenomenon of nyct.i.tropism had hitherto proved so baffling. It is indeed a difficult task to disentangle the full explanation of each given case in the vast complexity. It is, however, possible, by a process of judicious elimination, to reduce the difficulties which at first appear to be insurmountable.

In the periodic movement of plants there are several factors which are predominant, others being of minor importance. The important factors are the effects of light and darkness, of variation of temperature on differential growth, and of thermal variation on geotropic curvature.

For facility of treatment, I shall first take the three ideal types: (1) where the variation of light is the important factor, (2) where the movement is due to differential growth under variation of temperature, and (3) where thermal variation induces changes in geotropic curvature.

I shall then take up the movement of the leaf of _Mimosa_ where the combined effects of numerous factors give rise to a highly complex diurnal curve. There remains now the difficulty of discriminating the three types which approximate to the ideal.

DISCRIMINATING TESTS FOR CLa.s.sIFICATION.

_Predominant effect of light and darkness._--Turning first to the case where light exerts a predominant influence, the obvious test of keeping the plant in continuous darkness or continuous light is not practicable.

One would think that if the movement was due to periodic variation of light, such movement would disappear under constant light or darkness.

But owing to the persistence of after-effect, the periodic movement previously acquired is continued for a long time.

There is, however, another possibility of discrimination. The effect of variation of light will be most marked at the two periods, early in the morning when the light appears, and in the evening when it disappears.

In the tropics there is little twilight; in Calcutta, the sun rises in summer at about 5-30 a.m., and sets at 6-30 p.m. In winter the sun rises an hour later, and the sunset is an hour earlier. The average dawn may therefore be taken approximately at 6 a.m., and the average sunset at 6 p.m. Unlike the diurnal variation of temperature which is gradual, the change from light to darkness or from darkness to light is very abrupt.

If we succeed next in obtaining a continuous curve of the diurnal movement of the plant, the phototropic action would be evidenced by some flexures of the curve in the morning and towards evening.

The other two types of daily movement depend on the diurnal variation of temperature, and there is some difficulty in distinguishing the effect of variation of light from that of temperature, since both are connected with the appearance and disappearance of the sun.

_Diurnal variation of light and of temperature._--There are certain differences, however, which enable us to distinguish the two variations.

Light appears in the morning, say at 6 a.m., becomes most intense at noon; after 4 p.m. the light wanes, and darkness sets in quickly after 5 p.m. and remains persistent till next morning. The course of variation of temperature is somewhat different. The minimum temperature is attained in my green house at about 5 a.m. in summer, and at about 7 a.m. in winter. The maximum temperature is reached at about 3 p.m. in summer, and about 1 p.m. in winter. The range of daily variation in summer may be taken to be from about 23 C. to 34 C.; in winter it is from 16 C. to about 29 C. The above gives the normal variation and not the sudden fluctuations that occur during uncertain weather conditions.

The temperature remains constant for nearly an hour during the period of transition from falling to rising temperature, and _vice versa_. The average period of minimum temperature may be taken at 6 a.m., which I shall distinguish as the _thermal-dawn_. The average period for maximum temperature, the _thermal-noon_, is at 2 p.m. Variations from these average periods at different seasons do not amount to more than an hour.

The light-dawn and thermal-dawn are more or less coincident, while the thermal-noon is two hours later than the light-noon. A change in the diurnal curve of movement due to thermal variation will thus be detected at about 2 p.m. If the curve of daily movement of the plant-organ closely resemble the diurnal thermographic curve, there can then be no doubt of the causal relation of variation of temperature in the production of the periodic movement. Two different cla.s.ses of phenomena, as already stated, arise however from the variation of temperature, _thermonasty_ and _thermo-geotropism_. In the former, the movement is autonomous, and determined in relation to the plant; in the latter, the movement is related to the direction of external stimulus of gravity.

Further tests will be given later, to distinguish the phenomenon of Thermonasty from that of Thermo-geotropism.

I shall in the succeeding papers describe the princ.i.p.al types of diurnal movements as sketched above. The success of the investigation greatly depends on the elaboration of automatic apparatus of precision, which gives a continuous record of the diurnal movement of different plant organs. The description of this Nyct.i.tropic Recorder will be given in the next paper.

SUMMARY.

The obscurities in the nyct.i.tropic movement of plants arises from the presence of numerous complicating factors.

In the diurnal movement of plants the most important factors are the effects of light and darkness, of variation of temperature on differential growth, and of thermal variation on geotropic curvature.

These three cla.s.ses of phenomena may be discriminated from each other by the following tests. The effects of light and darkness are most p.r.o.nounced in the morning when light appears, and in the evening when light disappears. A p.r.o.nounced flexure in the diurnal curve at these periods indicates the dominant character of the phototropic action. The effect of light can also be distinguished from that of temperature from the fact that the period of maximum intensity of light, or _light-noon_, is about two hours earlier than the _thermal-noon_, at which the temperature is maximum.

A flexure of the diurnal curve about thermal noon, at which an inversion takes place from rise to fall of temperature, indicates the effect of temperature. The additional test of the effect of temperature is furnished by the close resemblance of the diurnal curve of the plant with the thermographic record for 24 hours.

Two different cla.s.ses of phenomena arise from variation of temperature--Thermonasty and Thermo-geotropism. In the former the movement is autonomous and determined by the differential growth-activity of the two sides of an anisotropic organ. In the latter the movement is not in relation to the plant but directed by the external stimulus of gravity.