20o beneath the horizon.  The terminal part, 10 mm. in length, had increased greatly in length during the 23 h. 30 m., viz., to an average of 26 mm.  Four control radicles became slightly geotropic after the 4 h. 10 m., and plainly so after the 5 h. 45 m.  Their mean length after the 23 h. 30 m. had increased from 10 mm. to 31 mm.  Therefore a slight cauterisation of the tip checks slightly the growth of the whole radicle, and manifestly stops the bending of that part which ought to bend most under the influence of geotropism, and which still continues to increase greatly in length.]

Concluding Remarks.—­Abundant evidence has now been given, showing that with various plants the tip of the radicle is alone sensitive to geotropism; and that when thus excited, it causes the adjoining parts to bend.  The exact length of the sensitive part seems to be somewhat variable, depending in part on the age of the radicle; but the destruction of a length of from less than 1 to 1.5 mm. (about 1/20th of an inch), in the several species observed, generally sufficed to prevent any part of the radicle from bending within 24 h., or even for a longer period.  The fact of the tip alone being sensitive is so remarkable a fact, that we will here give a brief summary of the foregoing experiments.  The tips were cut off 29 horizontally extended radicles of Vicia faba, and with a few exceptions they did not become geotropic in 22 or 23 h., whilst unmutilated radicles were always bowed downwards in 8 or 9 h.  It should be borne in mind that the mere act of cutting [page 541] off the tip of a horizontally extended radicle does not prevent the adjoining parts from bending, if the tip has been previously exposed for an hour or two to the influence of geotropism.  The tip after amputation is sometimes completely regenerated in three days; and it is possible that it may be able to transmit an impulse to the adjoining parts before its complete regeneration.  The tips of six radicles of Cucurbita ovifera were amputated like those of Vicia faba; and these radicles showed no signs of geotropism in 24 h.; whereas the control specimens were slightly affected in 5 h., and strongly in 9 h.

With plants belonging to six genera, the tips of the radicles were touched transversely with dry caustic; and the injury thus caused rarely extended for a greater length than 1 mm., and sometimes to a less distance, as judged by even the faintest discoloration.  We thought that this would be a better method of destroying the vegetative point than cutting it off; for we knew, from many previous experiments and from some given in the present chapter, that a touch with caustic on one side of the apex, far from preventing the adjoining part from bending, caused it to bend.  In all the following cases, radicles with uncauterised tips were observed at the same time and under similar circumstances, and they became, in almost every instance, plainly bowed downwards in one-half or one-third of the time