They all became (temp 20o - 21o C.) plainly geotropic in 8 h. 30 m., for they then stood at an average angle of 63o beneath the horizon. A rather greater length of the radicle is bowed downwards by geotropism than in the case of Vicia faba, [page 536] that is to say, rather more than 6 mm. as measured from the apex of the root-cap. Nine other radicles were similarly extended, three in damp peat and six in damp air, and dry caustic was held transversely to their tips during 4 or 5 seconds. Three of their tips were afterwards examined: in (1) a length of 0.68 mm. was discoloured, of which the basal 0.136 mm. was yellow, the apical part being black; in (2) the discoloration was 0.65 mm. in length, of which the basal 0.04 mm. was yellow; in (3) the discoloration was 0.6 mm. in length, of which the basal 0.13 mm. was yellow. Therefore less than 1 mm. was affected by the caustic, but this sufficed almost wholly to prevent geotropic action; for after 24 h. one alone of the nine cauterised radicles became slightly geotropic, being now inclined at 10o beneath the horizon; the eight others remained horizontal, though one was curved a little laterally.

The terminal part (10 mm. in length) of the six cauterised radicles in the damp air, had more than doubled in length in the 24 h., for this part was now on an average 20.7 mm. long. The increase in length within the same time was greater in the control specimens, for the terminal part had grown on an average from 10 mm. to 26.6 mm. But as the cauterised radicles had more than doubled their length in the 24 h., it is manifest that they had not been seriously injured by the caustic. We may here add that when experimenting on the effects of touching one side of the tip with caustic, too much was applied at first, and the whole tip (but we believe not more than 1 mm. in length) of six horizontally extended radicles was killed, and these continued for two or three days to grow out horizontally.

Many trials were made, by coating the tips of horizontally extended radicles with the before described thick grease. The geotropic curvature of 12 radicles, which were thus coated for a length of 2 mm., was delayed during the first 8 or 9 h., but after 24 h. was nearly as great as that of the control specimens. The tips of nine radicles were coated for a length of 3 mm., and after 7 h. 10 m. these stood at an average angle of 30o beneath the horizon, whilst the controls stood at an average of 54o. After 24 h. the two lots differed but little in their degree of curvature. In some other trials, however, there was a fairly well-marked difference after 24 h. between those with greased tips and the controls. The terminal part of eight control specimens increased in 24 h. from 10 mm. to a mean length of [page 537] 24.3 mm., whilst the mean increase of those with greased tips was 20.7 mm. The grease, therefore, slightly checked the growth of the terminal part, but this part was not much injured; for several radicles which had been greased for a length of 2 mm. continued to grow during seven days, and were then only a little shorter than the controls. The appearance presented by these radicles after the seven days was very curious, for the black grease had been drawn out into the finest longitudinal striae, with dots and reticulations, which covered their surfaces for a length of from 26 to 44 mm., or of 1 to 1.7 inch. We may therefore conclude that grease on the tips of the radicles of this Phaseolus somewhat delays and lessens the geotropic curvature of the part which ought to bend most.

Gossypium herbaceum.--The radicles of this plant bend, through the action of geotropism, for a length of about 6 mm. Five radicles, placed horizontally in damp air, had their tips touched with caustic, and the discoloration extended for a length of from 2/3 to 1 mm. They showed, after 7 h. 45 m. and again after 23 h., not a trace of geotropism; yet the terminal portion, 9 mm. in length, had increased on an average to 15.9 mm. Six control radicles, after 7 h.

Charles Darwin

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