Thus in all specimens examined, the experimentally determined geo-perceptive layer coincided with the 'starch sheath.' The theory of statoliths thus obtains strong support from an independent line of experimental investigation. The statolithic theory has been adversely criticised because in simpler organs the geotropic action takes place in the absence of statoliths. There is no doubt that the weight of the cell contents may in certain cases be effective in geotropic stimulation; it may nevertheless be true that "at a higher level of adaptation, the geotropically sensitive members of the plant-body are furnished with special geotropic sense-organs—a striking instance of anatomico-physiological division of labour."[38]
In the instances of Bryophyllum and Nymphæa given above, the geo-perceptive layer localised by means of the electric probe is definitely found to be the endodermis containing large sized starch grains.
INFLUENCE OF SEASON ON GEO-ELECTRIC RESPONSE.
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 Tropæolum declines rapidly in the course of a month from February to March (p. 454). The flowers of Nymphæa 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 prick of the probe: this, in vigorous specimens, is by an electric response of galvanometric negativity. But the response to prick 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 retardation of growth (p. 221). In the present investigation, the insensitive specimens were found to give abnormal positive electric response to the stimulus of prick made by the probe. The prick-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 Nymphæa, 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.