Part 28 (1/2)

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.

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