I shall now describe certain modifications in response, which result from the change of season and also from condition of high temperature.

Physiological reactions, generally speaking, are much affected by different seasons; thus the seedlings of _Scirpus Kysoor_ exhibit a very rapid rate of growth of 3 mm. per hour in August, but a month later the growth-rate declines to only 1 mm. per hour. I find similar depression of growth with the advance of season in seedlings of _Zea Mays_, where a very rapid fall in growth takes place in the course of a fortnight. The intensity of geotropic responses, both mechanical and electrical, of _Tropaeolum_ declines rapidly in the course of a month from February to March (p. 454). The flowers of _Nymphaea_ began to appear by the end of June when the flower stalks exhibited strong geo-electric response. But later in the season, by July and the beginning of August, the response underwent continuous decline, and by the end of August the response was nearly abolished.

Much time had to be spent in perfecting the apparatus, and it was not till the beginning of August that the investigations could be properly started; the responsive indications were, however, marked and definite, though relatively feeble compared to those obtained at the beginning of the season. The decline of the geo-electric response was to a certain extent also due to the prevailing high temperature.

_Effect of high temperature._--I shall in the next chapter describe experiments which show that geotropic response is diminished under rise of temperature. The specimens employed for localisation of geo-perceptive layer exhibited, as stated before, a decline of geo-electric response with the advance of the season. This may partly be due to unfavourable season, and partly to high temperature. In the middle of the season the responses were extremely feeble on warm days, but on cool mornings they became suddenly enhanced, to decline once more by the middle of the day. I could sometimes succeed in enhancing the sensitiveness by placing the specimen in a cold chamber. It thus appeared that certain internal change unfavourable for geo-perception takes place at high temperatures, and that the sensitive condition could sometimes be restored by artificial cooling. But later in the season, the internal change, whatever it may be, had proceeded too far, and artificial cooling did not restore the sensitiveness of the specimen.

What are the physico-chemical concomitants which distinguish insensitive specimens, in which the electric indications had declined almost to the vanishing point?

TEST OF INSENSITIVE SPECIMENS.

I shall now describe the various physico-chemical concomitants which accompany the condition of relative insensibility. I have found three different tests: the electric, the geotropic, and the microscopic, by which the sensitive could be distinguished from the insensitive condition. The following tests were made on insensitive specimens.

_Electric test: Experiment 187._--By the end of August the geo-electric indications given by the probe had, as stated before, almost disappeared. The tonic condition of the specimen, _below par_, was independently revealed by the response to p.r.i.c.k of the probe: this, in vigorous specimens, is by an electric response of galvanometric negativity. But the response to p.r.i.c.k in sub-tonic specimens is very different. I find that when the physiological condition of the tissue falls _below par_, the sign of response undergoes a reversal into one of _galvanometric positivity_. The same reversal under condition of sub-tonicity was also shown to take place in growth, where under the stimulus of light a positive acceleration took place, instead of normal r.e.t.a.r.dation of growth (p. 221). In the present investigation, the insensitive specimens were found to give abnormal positive electric response to the stimulus of p.r.i.c.k made by the probe. The p.r.i.c.k-effect in fact often gave me previous indication as to the suitability of the particular specimen for exhibition of geo-electric response.

_Test of geotropic reaction: Experiment 188._--I took four different specimens of _Bryophyllum_ and _Nymphaea_, and held them horizontal.

These plant organs had, earlier in the season, exhibited very strong geotropic effect, the shoot curving up through 90 in the course of ten hours or less. But these specimens obtained later in the season exhibited very feeble curvature, which hardly amounted to 10 degrees, even after prolonged exposure to geotropic action for 24 hours.

_Test of microscopic examination._--I next made sections of _Bryophyllum_ and _Nymphaea_ and on examining them under the microscope discovered certain striking changes. A fortnight ago the group of large starch grains stained with iodine were the most striking feature of the starch sheath. But now these starch grains could not be found in any of the numerous specimens examined. The presence of the starch grains thus appears to be a.s.sociated with the sensitiveness of the perceptive layer.

REACTION AT LOWER SIDE OF THE ORGAN.

There remains now the important question of the physiological change induced on the lower side of the horizontally laid shoot. The physiological reaction of two sides of the organ must be different, since the upper side exhibits contraction and the lower side expansion.

It may be urged that the effect of one of the two sides might result from the pa.s.sive yielding to the definite reaction induced on the opposite side. Investigation by the electric method enables us, however, to discriminate the two reactions from each other, since the electric response characteristic of the induced physiological change takes place in the organ, even under condition of restraint by which movement is prevented. We shall therefore investigate the geo-electrical reaction on the lower side of the securely held organ, and find out whether the induced electric change undergoes any variation in different layers from below upwards. There are two different ways in which the electric explorations of the lower side of the organ may be carried out. In the first method, the probe is introduced from below, and successive readings for geo-electric response taken as the probe enters the organ by successive steps. It is understood that the true geotropic effect is found from difference of galvanometer readings in vertical and horizontal positions. In the second method, the probe is introduced from above, and successive readings for the response taken for different positions of the probe as it enters the organ from the upper side and comes out ultimately at the lower side. This I shall call the METHOD OF TRANSVERSE PERFORATION. The intrusion of the probe on the upper side gives, as we have seen, increasing negative deflection of the galvanometer which reaches a maximum at the perceptive layer. Pa.s.sage of the probe to still greater depths give deflections which decline to zero. But when the probe comes within the influence of the perceptive layer of the under side, the electric indication, as we shall presently find, undergoes a reversal.

ELECTRIC EXPLORATION OF THE LOWER SIDE OF THE ORGAN.

I shall first describe the results obtained from the first method, the probe entering the organ from the lower side.

_Experiment 189._--The investigation was carried out with the stem of _Bryophyllum_, and the flower stalk of _Nymphaea_. The probe was made to enter the organ through 04 mm. and the geo-electric effect found, on rotation of the flower stalk of _Nymphaea_ from the vertical to the horizontal, was a deflection of +6 divisions of the galvanometer. _The change induced at the lower side by geotropic stimulus is thus galvanometric positivity, indicative of enhancement of turgor, and, of expansion._ Intrusion of the probe through 06 mm. gave rise to an increased positive geo-electric response. That the sign of electric response depended on the relation of the side of the organ to the vertical lines of gravity was demonstrated by alternate rotation of the plant through +90 and -90, the probe remaining at a definite position.

Rotation through +90 brought A above, and rotation through -90 brought A below. When the probe was in the _up_ position the geo-electric response was negative, but when rotation through -90 brought it _below_, the response became positive. Thus with an identical contact in the plant, the electric response underwent reversal from negative to positive. This will be understood from the following table.

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

Position of the

Galvanometer

Galvanometer

probe inside

deflection:

deflection:

the organ.

A _above_.

A _below_.

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

04 mm.

-8 divisions.

+5 divisions.

06 mm.

-16 "

+10 "

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

It will thus be seen that physiological change induced at any point is modified by its relation to vertical lines of gravity. When the point is above, the induced change is _negative_, when below, the induced change is _positive_.

I shall next describe the variation of effect at different layers of the under side of the organ.

_Experiment 190._--A complete set of readings of the geo-electric reaction at different layers of the organ was taken, as the probe entered the lower side by successive steps of 02 mm. The following table gives the results obtained with a specimen of _Nymphaea_.

TABLE XLIII.--ELECTRIC EXPLORATION OF DIFFERENT LAYERS ON THE LOWER SIDE OF THE ORGAN (_Nymphaea_).

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

Position of the

Galvanometer

probe.

deflection.

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

Surface

2 divisions.

02 mm.

4 "

04 "

8 "

06 "

16 "

08 "

20 "

10 mm.

32 "

12

16 "

14 "

12 "

16 "

4 "

18 "

0 "

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

It is thus seen that just as in the upper so also in the lower side, the electric variation undergoes at first an increase which attains a maximum; beyond this point the electric change undergoes a rapid decline. The induced electric change on the upper and lower sides are, however, different, galvanometric _negativity_ in one case and _positivity_ in the other.

The maximum galvanometric _negativity_ of the upper side was found to occur at the geo-perceptive layer. We may next inquire about the anatomical characteristic of the layer in the lower side of the organ which exhibits the maximum galvanometric _positivity_. Microscopic section of the specimen employed in the above experiment showed the particular layer to be the starch crescent which lies above the vascular bundle. Thus the same geotropic layer which when placed above shows the maximum galvanometric negativity, exhibits maximum positivity when placed below.

METHOD OF TRANSVERSE PERFORATION.

_Experiment 191._--I next carried out a complete exploration of the interior of the organ along the diameter. The probe started from the upper surface, and came out at the lower by successive steps of 02 mm., the corresponding geo-electric effects being observed at each step. It has to be borne in mind that the successive readings were obtained by rotation from vertical to +90 (A above); the rotation was never carried out in the negative direction through -90. But the probe entering from above pa.s.sed the central axis, and entered a region where the galvanometric indication was transformed from negative to positive. The following table gives the results obtained with the flower stalk of _Nymphaea_.

TABLE XLIV.--SHOWING THE INDUCED GEO-ELECTRIC DISTRIBUTION ACROSS THE FLOWER STALK OF _Nymphaea_ (diameter = 68 mm.)

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

Position of

Galvanometer

probe.

deflection.

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

Surface ...

- 10 divisions.

02 mm. ...

- 26 "

04 " ...

- 40 "

06 " ...

- 50 "

08 " ...

- 62 "

10 " ...

- 72 "

12 " ...

- 88 "

14 " ...

-108 "

16 " ...

- 72 "

18 " ...

- 44 "

20 " ...

- 30 "

22 " ...

- 18 "

24 " ...

- 10 "

26 " ...

- 5 "

28 " ...

- 2 "

30 " ...

0 "

32 " ...

0 "

34 " ...

0 "

36 mm. ...

0 divisions.

38 " ...

0 "

40 " ...

0 "

42 " ...

+ 2 "

44 " ...

+ 4 "

46 " ...

+ 5 "

48 " ...

+ 11 "

50 " ...

+ 22 "

52 " ...

+ 38 "

54 " ...

+ 46 "

56 " ...

+ 39 "

58 " ...

+ 32 "

60 " ...

+ 24 "

62 " ...

+ 18 "

64 " ...

+ 12 "

66 " ...

+ 6 "

68 " ...

+ 3 "

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

[Ill.u.s.tration: FIG. 177.--Curve of geo-electric excitation in different layers of _Nymphaea_. Ordinate represents geo-electric excitation; abscissa, distance from upper surface of flower stalk. The diagrammatic section underneath shows the position of geo-perceptive layer (starch-sheath) corresponding to maximum induced galvanometric negativity and positivity on the two sides.]

[Ill.u.s.tration: FIG. 178.--The curve of geo-electric excitation in different layers of _Bryophyllum_.]

A curve constructed from the data given above is seen in figure 177. The diameter of the flower stalk was 68 mm. The negative geo-electric reaction is seen to undergo an increase till it attains a climax at the depth of 14 mm. It then undergoes a continuous diminution till it becomes zero at the depth of 3 mm.; this neutral zone extends through 1 mm. When the probe enters a depth of 42 mm. measured from the upper side, it enters a region affected by the perceptive layer situated on the under side, the opposite physiological reaction being indicated by induced electric change of galvanometric positivity. This positivity reaches a climax at a depth of 54 mm. measured from the upper side, and 14 mm. when measured from the lower side. The points of maximum positivity and negativity are situated symmetrically on the opposite sides of the organ. The electric variation of maximum positivity on the lower side is comparatively feeble, less than half the corresponding maximum negativity on the upper side. Microscopic section showed that the geo-perceptive layers were the same as the starch-crescents.

_Experiment 192._--I carried out similar experiments with the shoot of _Bryophyllum_. The results are given in Table XLV; the curve of the electric distribution along the diameter is seen in figure 178. The characteristics of this curve are the same as that of _Nymphaea_. The maximum galvanometric negativity occurred at the depth of 06 mm., and of positivity at a corresponding point on the opposite side.

TABLE XLV.--SHOWING INDUCED GEO-ELECTRIC DISTRIBUTION ACROSS THE STEM OF _Bryophyllum_ (diameter = 36 mm.).

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

Position of

Galvanometric

probe.

deflection

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

Surface

0 divisions.

02 mm.

-24 "

04 "

-45 "

06 "

-63 "

08 "

-21 "

10 "

- 9 "

12 "

- 6 "

14 "

- 3 "

16 "

0 "

18 "

0 "

20 "

0 divisions.

22 "

0 "

24 "

+ 3 "

26 "

+ 4 "

28 "

+ 9 "

30 "

+36 "

32 "

+21 "

34 "

+ 9 "

36 "

0 "

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

Microscopic examination showed that the electric maxima in _Bryophyllum_ coincided with the diametrically opposite points in the continuous endodermic ring. In _Bryophyllum_ as in _Nymphaea_, the excitatory galvanometric negativity of the upper geo-perceptive layer is greater than the induced positivity of the lower layer in the ratio of about 2:1. But in a depressed condition of the tissue, the excitatory reaction is the first to disappear and the positive reaction persists, though with diminished intensity.

The geo-electric distribution in vigorous specimens seems to indicate that under the stimulus of gravity a marked excitatory reaction (contraction) takes place in the layer of cells contiguous to the upper geo-perceptive layer, and, a less marked positive reaction (expansion) occurs in layers contiguous to the lower perceptive layer.

It is remarkable that physiological reaction of opposite kinds should occur on the upper and lower sides of an organ under the identical stimulus of gravity. The difference of reaction may conceivably be connected with the fact that the vertical lines of gravity enter by the upper, and leave by the lower side of the organ. The statolithic particles rest on the inner tangential walls of the perceptive cells of the upper layer, and on the outer tangential walls of the lower layer.

Similar difference of physiological reactions of a polar character are also known in responses of plants under the action of an identical electric current; here with different ionic distributions, contraction takes place at the kathode, and expansion at the anode.

The geo-electric reactions that have been described were obtained under unfavourable conditions of climate and of temperature. But under better conditions the reaction becomes very greatly enhanced, as would appear from the following account of results which I obtained on two separate occasions in the beginning of August. The season had not become quite as unfavourable as towards the end of the month, but the prevailing sultry weather had caused great depression of the geo-electric excitability.

On the first occasion referred to, thunderstorm had broken out at night, and it was refreshingly cool in the morning. It was with the utmost surprise that I noted the astonishing violence of the geo-electric response which the plants gave that morning; the maximum response hitherto obtained was about 100 divisions of the galvanometer scale; but on the present occasion the displacement of the plant, from vertical to horizontal position, induced responsive deflection so great that the galvanometer spot of light flew off the scale of 3,000 divisions. I was at first incredulous of the results and wasted the valuable occasion in trying to discover some hidden source of error. Subsequent tests showed that my misgivings were groundless, and that the extraordinary large deflection was really due to geo-electric reaction. On the second favourable occasion, which lasted for three hours (during the cool hours of the morning), I was able to secure a number of important observations. Thus displacement of the flower stalk of _Nymphaea_ through +90 was immediately followed by geo-electric response, the deflection being about 3,000 divisions of the scale. The latent period hardly exceeded a second; the return of the plant to the vertical position was quickly followed by electric recovery which was complete. The above results were obtained with the same specimen time after time without a single failure. The successive responses showed no sign of fatigue.

Another remarkable effect was noticed during gradual increase of the angle of inclination. Nothing happened till a critical angle was reached, which was roughly estimated to be about 33; when this critical angle was exceeded by a single degree, there was a sudden precipitation of geo-electric response. The experiments were repeated time after time with the identical result. It appeared as if some frictional resistance obstructed the displacement of the geotropic particles acc.u.mulated at the basal end of the cell, and it was not till the organ had been tilted beyond 33 that this resistance to sliding was overcome.

SUMMARY.

The electric distribution induced in an organ under the stimulus of gravity may be mapped out by means of an exploring Electric Probe.

The induced galvanometric negativity of the upper side of an organ (indicative of excitation) undergoes variation in different layers of the organ. The excitatory reaction attains a maximum value at a definite layer, beyond which there is a decline.

The geo-perceptive layer is experimentally localised by measuring the depth of intrusion of the probe for maximum deflection of galvanometric negativity.

The geo-perceptive layer thus determined is found to be the starch sheath which contains a number of large-sized starch grains.

The power of geo-perception undergoes seasonal variation. It is also lowered by high temperature.

The geo-electric response undergoes decline with growing sub-tonicity of the specimen; such specimens exhibit abnormal positive electric response under the stimulus of p.r.i.c.k and feeble curvature under geotropic stimulus. The large-sized starch-grains, normally observed in the endodermis, are found to disappear in specimens which have become geo-electrically insensitive.

The electric response of the lower side of the organ to gravitational stimulus is of opposite sign to that of the upper side. The electric distribution on the lower side exhibits variations in different layers, the maximum positivity occurring at the perceptive layer. In vigorous specimens the excitatory negative electric change on the upper side is greater than the positive electric change on the lower side. Depressed condition of the tissue is attended by a relatively greater decline of the negative in comparison with the positive.