Part 31 (1/2)
1. The organs are sensitive to the stimulus of gravity and the periodic movements are brought about by variation of geotropic curvature under change of temperature.
2. The movement is not confined to growing organs, but is also exhibited by organs which are fully grown and even by rigid trees.
3. The periodic movement is closely related to the diurnal variation of temperature. Fall of temperature from thermal-noon (about 2 p.m.) to thermal-dawn (about 6 a.m.) is attended by a movement of erection; rise of temperature from thermal-dawn to thermal-noon is followed, on the other hand, by a reverse movement of fall.
That the movement is primarily due to variation of temperature will be demonstrated in two different ways:
(_a_) by the change of normal rhythm of movement by artificial transpositions of periods of maximum and minimum temperature, and
(_b_) by the abolition of periodic movement through maintenance of constant temperature.
That the phenomenon is not nastic, but paratonic will be demonstrated:--
(_a_) by the reversal of closure into opening movement and _vice versa_, in consequence of inversion of the plant upside down, and
(_b_) by the diurnal variation of torsional movement, the direction of which is dependent on the directive action of the stimulus of gravity.
I shall now describe the diurnal movement of various geotropically curved plant-organs; the most striking example of this is furnished by the 'Praying' Palm of Faridpore, already described. I shall here recapitulate some of the important features connected with the phenomenon.
DIURNAL MOVEMENT OF PALM TREES.
Movements similar to that of the Faridpore Palm (p. 12) are found in other Palm trees growing at an inclination from the vertical. I reproduce once more the diurnal curve given by the Sijberia Palm together with the curve of daily thermal variation (Fig. 199). It will be seen that the two curves resemble each other so closely that the curve of movement of the tree is practically a replica of the thermographic record. There can therefore be no doubt of the movement being brought about by variation of temperature; rise of temperature is attended by the movement of fall of the tree and _vice versa_. The record was commenced at noon; the temperature rose till the maximum was reached at about 3 p.m. and the tree also reached its lowest position at 3-45 p.m., the lag being 45 minutes. The temperature fell continuously after the maximum at 3 p.m., to the minimum at 6 a.m. next morning. In response to the falling temperature, the tree exhibited a movement of erection. The temperature rose after 6 a.m. and the movement of the tree became reversed from ascent to descent.
[Ill.u.s.tration: FIG. 199.--Diurnal record of the Sijberia Palm. Upper curve gives variation of temperature, and the lower curve the movement of the tree.]
I have already shown: (1) that the diurnal movement just described is due to physiological reaction, and that the movement is abolished at the death of the plant; (2) that light has little or no effect, since the thick bark and bases of leaves screen the living tissue from the action of light; (3) that transpiration has practically no effect on the periodic movement, since such movement takes place in other plants completely immersed under water; thus _Ipomoea aquatica_, a water plant, kept under water, gave the normal diurnal curve similar to that of the palm. The modifying effect of transpiration was in this case, completely excluded. I obtained similar effect with geotropically curved stem of _Basella cordifolia_ (p. 25); (4) that the weight of the plant-organ as such, has little effect on the diurnal curve, since an inverted plant continues for a few days to exhibit the periodic movement, in spite of the antagonistic effect of weight. A different experiment will be described (see p. 582) where the effect of weight was completely neutralised and the plant-organ gave, nevertheless, the normal diurnal curve.
[Ill.u.s.tration: FIG. 200.--Diurnal record of inclined palm tree, of geotropically curved proc.u.mbent stem of _Tropaeolum_ and the dia-geotropic leaf of palm. Note general similarity between diurnal curve of plants and the thermographic record.]
I have also shown that the diurnal movement is determined by the modifying influence of temperature on geotropic curvature. Rise of temperature opposes or neutralises the geotropic curvature; fall of temperature, on the other hand, accentuates it. The particular diurnal movement was not confined to the palm trees, but was exhibited by all plant-organs subjected to the stimulus of gravity.
DIURNAL MOVEMENT OF PROc.u.mBENT STEMS AND OF LEAVES.
[Ill.u.s.tration: FIG. 201.---Diurnal records of leaves of _Dahlia_, _Papaya_ and _Croton_.]
_Experiment 210._--In order to demonstrate the continuity of the phenomenon of diurnal movement I took various stems growing in water or land for my experiment. The plants were laid horizontally, till the stems bent up and a.s.sumed the stable position of geotropic equilibrium.
In figure 200 is given records of the inclined palm tree, of proc.u.mbent stem of _Tropaeolum_, and the leaf of the palm tree. The very close relation between the temperature-variation and the movement of different plant-organs is sufficiently obvious.
I shall next give a series of diurnal records of leaves of different plants such as those of _Dahlia_, _Papaya_ and _Croton_ (Fig. 201). In these also fall of temperature induces an up-movement while rise of temperature causes a fall of the leaf. I shall presently refer to the 'personal equation' by which the record of one plant is distinguished from another.
CONTINUOUS DIURNAL RECORD FOR SUCCESSIVE THERMAL NOONS.
_Experiment 211._--The diurnal record given above, was taken from ordinary noon at 12 o'clock to noon next day. The diurnal curve becomes much simplified if the record be taken from _thermal-noon_ (at about 2 p.m.) to the thermal noon next day. The plant-organ becomes erected during falling temperature from thermal-noon to thermal-dawn next morning, and undergoes a fall during rise of temperature from thermal-dawn to thermal-noon. The subsequent diurnal records will therefore be given for 24 hours commencing with 2 p.m. In figure 202 is given diurnal records of geotropically curved stem of _Tropaeolum_ and the leaf of _Dahlia_ for two days in succession.
The thermal record shows that there was a continuous fall of temperature from thermal-noon at 2 p.m. to the thermal-dawn at 6 a.m. next morning, that is to say, for 16 hours. Rise of temperature through the same range occurred in 8 hours from 6 a.m. till 2 p.m. The average rate of rise of temperature was thus twice as quick as that of fall. This is clearly seen from the slopes of thermal curve during thermal ascent and descent.
The record of the movement of the plant shows a striking parallelism; the different plant-organs became erected from thermal-noon to thermal-dawn, and underwent a fall from thermal-dawn to thermal-noon.
The descent of the curve is, as in the case of thermal curve, relatively more abrupt. The records on two successive days are very similar, the slight difference being due to the physiological depression consequent on prolonged maintenance of the plants in a closed chamber.