In the following experiments, the tips of young horizontally extended radicles were just touched with a stick of dry caustic; and this was held transversely, so that the tip might be cauterised all round as symmetrically as possible.  The radicles were then suspended in a closed vessel over water, kept rather cool, viz., 55o — 59o F. This was done because we had found that the tips were more sensitive to contact under a low than under a high temperature; and we thought that the same rule might apply to geotropism.  In one exceptional trial, nine radicles (which were rather too old, for they had grown to a length of from 3 to 5 cm.), were extended horizontally in damp friable earth, after their tips had been cauterised and were kept at too high a temperature, viz., of 68o F., or 20o C. The result in consequence was not so striking as in the subsequent cases for although when after 9 h. 40 m. six of them were examined, these did not exhibit any geotropic bending, yet after 24 h., when all nine were examined, only two remained horizontal, two exhibited a trace of geotropism, and five were slightly or moderately geotropic, yet not comparable in degree with the control specimens.  Marks had been made on seven of these cauterised radicles at 10 mm. from the tips, which includes [page 531] the whole growing portion; and after the 24 h. this part had a mean length of 37 mm., so that it had increased to more than 3 ½ times its original length; but it should be remembered that these beans had been exposed to a rather high temperature.

Nineteen young radicles with cauterised tips were extended at different times horizontally over water.  In every trial an equal number of control specimens were observed.  In the first trial, the tips of three radicles were lightly touched with the caustic for 6 or 7 seconds, which was a longer application than usual.  After 23 h. 30 m. (temp. 55o — 56o F.) these three radicles,

Fig. 196.  Vicia faba:  state of radicles which had been extended horizontally for 23 h. 30 m.; A, B, C, tips touched with caustic; D, E, F, tips uncauterised.  Lengths of radicles reduced to one-half scale, but by an accident the beans themselves not reduced in the same degree.

A, B, C (Fig. 196), were still horizontal, whilst the three control specimens had become within 8 h. slightly geotropic, and strongly so (D, E, F) in 23 h. 30 m.  A dot had been made on all six radicles at 10 mm. from their tips, when first placed horizontally.  After the 23 h. 30 m. this terminal part, originally 10 mm. in length, had increased in the cauterised specimens to a mean length of 17.3 mm., and to 15.7 mm. in the control radicles, as shown in the figures by the unbroken transverse line; the dotted line being at 10 mm. from the apex.  The control or uncauterised radicles, therefore, had actually grown less [page 532] than the cauterised; but this no doubt was accidental, for radicles of different ages grow at different rates,