Vicia faba.—­Radicles of this plant were extended horizontally either over water or with their lower surfaces just touching it.  Their tips had previously been cut off, in a direction as accurately transverse as could be done, to different lengths, measured from the apex of the root-cap, and which will be specified in each case.  Light was always excluded.  We had previously tried hundreds of unmutilated radicles under similar circumstances, and found that every one that was healthy became plainly geotropic in under 12 h.  In the case of four radicles which had their tips cut off for a length of 1.5 mm., new root caps and new vegetative points were re-formed after an interval of 3 days 20 h.; and these when placed horizontally were acted on by geotropism.  On some other occasions this regeneration of the tips and reacquired sensitiveness occurred within a somewhat shorter time.  Therefore, radicles having their tips amputated should be observed in from 12 to 48 h. after the operation.

Four radicles were extended horizontally with their lower surfaces touching the water, and with their tips cut off for a length of only 0.5 mm.:  after 23 h. three of them were still horizontal; after 47 h. one of the three became fairly geotropic; and after 70 h. the other two showed a trace of this action.  The fourth radicle was vertically geotropic after 23 h.; but by an

* ‘Arbeiten des Bot.  Instituts in Würzburg,’ Heft. iii. 1873, p. 432. [page 525]

accident the root-cap alone and not the vegetative point was found to have been amputated; so that this case formed no real exception and might have been excluded.

Five radicles were extended horizontally like the last, and had their tips cut off for a length of 1 mm.; after 22-23 h., four of them were still horizontal, and one was slightly geotropic; after 48 h. the latter had become vertical; a second was also somewhat geotropic; two remained approximately horizontal; and the last or fifth had grown in a disordered manner, for it was inclined upwards at an angle of 65o above the horizon.

Fourteen radicles were extended horizontally at a little height over the water with their tips cut off for a length of 1.5 mm.; after 12 h. all were horizontal, whilst five control or standard specimens in the same jar were all bent greatly downwards.  After 24 h. several of the amputated radicles remained horizontal, but some showed a trace of geotropism, and one was plainly geotropic, for it was inclined at 40o beneath the horizon.

Seven horizontally extended radicles from which the tips had been cut off for the unusual length of 2 mm. unfortunately were not looked at until 35 h. had elapsed; three were still horizontal, but to our surprise, four were more or less plainly geotropic.