and the growth of different individuals is likewise affected by unknown causes.  The state of the tips of these three radicles, which had been cauterised for a rather longer time than usual, was as follows:  the blackened apex, or the part which had been actually touched by the caustic, was succeeded by a yellowish zone, due probably to the absorption of some of the caustic; in A, both zones together were 1.1 mm. in length, and 1.4 mm. in diameter at the base of the yellowish zone; in B, the length of both was only 0.7 mm., and the diameter 0.7 mm.; in C, the length was 0.8 mm., and the diameter 1.2 mm.

Three other radicles, the tips of which had been touched with caustic curing 2 or 3 seconds, remained (temp. 58o — 59o F.) horizontal for 23 h.; the control radicles having, of course, become geotropic within this time.  The terminal growing part, 10 mm. in length, of the cauterised radicles had increased in this interval to a mean length of 24.5 mm., and of the controls to a mean of 26 mm.  A section of one of the cauterised tips showed that the blackened part was 0.5 mm. in length, of which 0.2 mm. extended into the vegetative point; and a faint discoloration could be detected even to 1.6 mm. from the apex of the root-cap.

In another lot of six radicles (temp. 55o — 57o F.) the three control specimens were plainly geotropic in 8 ½ h.; and after 24 h. the mean length of their terminal part had increased from 10 mm. to 21 mm.  When the caustic was applied to the three cauterised specimens, it was held quite motionless during 5 seconds, and the result was that the black marks were extremely minute.  Therefore, caustic was again applied, after 8 ½ h., during which time no geotropic action had occurred.  When the specimens were re-examined after an additional interval of 15 ½ h., one was horizontal and the other two showed, to our surprise, a trace of geotropism which in one of them soon afterwards became strongly marked; but in this latter specimen the discoloured tip was only 2/3 mm. in length.  The growing part of these three radicles increased in 24 h. from 10 mm. to an average of 16.5 mm.

It would be superfluous to describe in detail the behaviour of the 10 remaining cauterised radicles.  The corresponding control specimens all became geotropic in 8 h.  Of the cauterised, 6 were first looked at after 8 h., and one alone showed a trace [page 533] of geotropism; 4 were first looked at after 14 h., and one alone of these was slightly geotropic.  After 23 — 24h., 5 of the 10 were still horizontal, 4 slightly, and 1 decidedly, geotropic.  After 48 h. some of them became strongly geotropic.  The cauterised radicles increased greatly in length, but the measurements are not worth giving.