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Show 164 SENSITIVENESS OF THE APEX CHAP. III. of various sizes of the same card-like paper,. also with bits of thin glass and rough cinders, affixed w1th shellac to one side of the apex. Rather large <lrops of the dissolved shellac were also placed on them and allowed t set into hard beads. The specimens were subjected t~ various temperatures between 60° and 72° F:, more commonly at about the latter. But. out of tlns ~onsidentble number of trials only 5 rachcles wore plmnly bent, and 8 others slightly or .c~cn doubtfully: from the attached 0 bj ects ; the rema1mng 46 not. bcmg at all affected. It is therefore clear that the tips. ~f the radicles of this Phaseolus are much less sensitive to contact tht"tn are those of the bean or pea. We thought that they might be sensitive to han~er pressure, but after several trials we could ~ot dev1se any method for pressing harder on one s1do of .the apex than on the other, without a~ the same tune offering mechanic<"tl resistance to 1 ts growth. We therefore tried other irritants. rrhe tips of 13 radicles, dried with blotting-pa~Cl', were thrice touched or ju t rubbe<l on one s.1de with dry nitmte of silver. rrhcy were 0 rubbe.d thnce, because we supposed from the forcgomg trwJs, that the tips were not highly sensitive. After 24 h. the tips were found greatly blackene<l; G were blackened equally all round, so that no curvttture to any one side could be expected; G were much blac~cened. 011 one side for a leno·th of about -fu-th of an meh, and this lenoth became curved at right angles towards ~he 0 . d 0 ,. J10' blackened surface, the curvttturc after war s mm easl b in several instances until little hooks were fonned. It was manifest that the black one l side was so mu.ch iniured that it could not grow, whilst the opposlt(} sid· e continueil. to oTow. 0 no a 1o ne ou t of these 13 radiclcs became cu~ved from the LlackeneJ side, the CnAP. III. OF THE RADICLJ£ OF PIIAREOLU, . 1G:J curvature extending for some little distance abon• the apex. After the experience thus gain d, the tips of six almost dry radicles were once touched with the dry caustic on one side; and after an interval of 10 m. were allowed to enter wu.tcr, which was kept at a temperature of G5°-G7° }..,. 'l'hc r ult wa that after an interval of 8 h. a minute blacki.·h speck coultl just be distinguished on one side of the apex of :fiyf' of these radicles, all of which became curv d towards the opposite side-in two ca es at about an angle of 45°-in two other cas s at nearly a rectangle-and in the fifth case at above a rectangle, so that the apex was a little hooked; in this latter case the black mark was rather larger than in the others. After 24: h. from the application of the caustic, the curvature of three of these rauicles (including the hooked one) had diminished; in the fourth it remained the same, and in the fifth it had increase l, the tip being now hooked. It has been said that after 8 h. black specks could be seen on one side of the apex of five of the six radicles ; on the sixth the sp ek, w hieh was extreme! y minute, was on the actual apex fLntl therefore central; and this radicle alone dicl not become curved. It was therefore again touched on one side with caustic, aml a~ter 15 h. 30m. was found curved from the perpendiCular and from the blackened side at an an o-le of 34" . . 0 ' whiCh mcreased in nine additional hours to 54°. It is therefore certain that the apex of the radicle of this Phaseblus is extremely sensitive to caustic, more so than that of the bean, though the latter is ~ar more sensitive to pressure. In the experiments J~st given, the curvature from the slightly cauterised Slde of the tip, extended along the radicle for a length of nearly 10 mm.; whereas in the first set |
