with only slightly discoloured tips were bent down rectangularly; the other five were not in the least affected, but several of them had grown rather tortuously, though still in a horizontal plane.  The eight cauterised radicles which had at first a mean length of .36 inch, after 9 h. had increased to a mean length of .79 inch; and after 24 h. 40 m. to the extraordinary mean length of 2 inches.  There was no plain difference in length between the five well cauterised radicles which remained horizontal, and the three with slightly cauterised tips which had become abruptly bent down.  A few of the control radicles were measured after 25 h., and they were on an average only a little longer than the cauterised, viz., 2.19 inches.  We thus see that killing the extreme tip of the radicle of this plant for a length of about 0.5 mm., though it stops the geotropic bending of the upper part, hardly interferes with the growth of the whole radicle.

In the same box with the 15 control specimens, the rapid geotropic bending and growth of which have just been described, there were six radicles, about .6 inch in length, extended horizontally, from which the tips had been cut off in a transverse direction for a length of barely 1 mm.  These radicles were examined after 9 h. and again after 24 h. 40 m., and they all remained horizontal.  They had not become nearly so tortuous as those above described which had been cauterised.  The radicles with their tips cut off had grown in the 24 h. 40 m. as much, judging by the eye, as the cauterised specimens.

Zea mays.—­The tips of several radicles, extended horizontally in damp air, were dried with blotting-paper and then touched in the first trial during 2 or 3 seconds with dry caustic; but this was too long a contact, for the tips were blackened for a length of rather above 1 mm.  They showed no signs of geotropism after an interval of 9 h., and were then thrown away.  In a second trial the tips of three radicles were touched for a shorter time, and were blackened for a length of from 0.5 to 0.75 mm.:  they all remained horizontal for 4 h., but after 8 h. 30 m. one of them, in which the blackened tip was only 0.5 mm. in length, was inclined at 21o beneath the horizon.  Six control radicles all became slightly geotropic in 4 h., and strongly so after 8 h. 30 m., with the chief seat of curvature generally between 6 or 7 mm. from the apex.  In the cauterised specimens, the terminal growing part, 10 mm. in length, increased during [page 540] the 8 h. 30 m. to a mean length of 13 mm.; and in the controls to 14.3 mm.

In a third trial the tips of five radicles (exposed to a temp. of 70o — 71o) were touched with the caustic only once and very slightly; they were afterwards examined under the microscope, and the part which was in any way discoloured was on an average .76 mm. in length.  After 4 h. 10 m. none were bent; after 5 h. 45 m., and again after 23 h. 30 m., they still remained horizontal, excepting one which was now inclined