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Show 19-1 SUMMARY OF CIIAP'l'ER CnAP. nr. very sl 1. g ht, and the adJ' oinin. rbr upper part . bond.s away from the cauterised point, with more certam_ty m most cases tha n f ro m an obJi ect attached on one s1cle. Here I· t 0 b Vl·O US1 Y I·s not the mere tou.c h, but the e· ffect pr.o d uce d b Y the caustic, which Induces the tip to 1 . . . t . transm1' t some I'nfluonce to the ac JOmmg p. ar ,. causmg I. t t o b end a'"·• ay · If one side . of . the t1p 1s badly . . d . 1... lllJlUe OI \. 1·lled by the cau. stic, It cea· ses to growh, whilst the opposite side continues growmg ; a?~ t e lt . th t the tip itself bends towards the InJured re·asu IS a 1 d · · d ften becomes completely hookot ; an It 1s SI e an 0 d' · · t remarkable that in this case th~ a JOmm.g upper par tlo es no t b enu.1 . The stimulus IS. too powerful or the shock too great for the proper I~fluence to be trans· mitted from the tip. We have stnctly analo~ous ca.ses with Drosera, Dionrea and Pinguicula, with. whiCh lants a too powerful stimulus docs not exmte the fentacles to become incurved, or the lobes t o close, or the margin to be folded in wards. . . With respect to the degree of sensitiveness of the a ex to contact under favourable condit ions, we. ~ave s~en that with Vicia Jaba a little square of wntm,g· paper affixed with shellac sufficed to cause mo, e· ment. as did on one occasion a square of merely damp~cl goldboaters' skin, but it acted very. slowly. Short bits of moderately thick bristle (of whwh mea· surements have b een gi. ven) a ffi x eel with bO 'Um-waterf acted in only three out of eleven trial~, an<~ beads ol dried shellac under 2~oth of a grain m weight a:te< Only twice in nine cases; so that here we . ~tve nearly reached the minimum o£ . necessary. .ll 'Tl at · tl.on The apex therefore, is much less sensitive o · ' f these are pressure than the glands of Drosera, . or f b: tle ·:tffectecl by far thinner objects than bits o ns. ' <a nd by a very much less wm• ght t h an 2o1 o tl1 of a O'nllll· b CHAP. III. SUMMARY OF CHAPTER. ID5 But the most interesting evidence of tho delicate sensitiveness of the tip of the radicle, was affordecl by its power of discriminating between equal-sized squares of card-like and very thin paper, when these were attached on opposite sicles, as was observed with the radicles of the bean and oak. When radicles of the bean are extended horizontally with squares of card attachecl to the lower sides of their tips, the irritation thus caused was always conquered by geotropism, which then acts under the most favourable conditions at right angles to the radicle. But when objects were attached to the radicles of any of the above-named genera, suspencled vertically, the irritation conquered geotropism, which latter power ~t first acted obliquely on the radicle ; so that thf:l 1m~ediate irritation from the attachocl object, aided by Its after-effects, prevailed and caused the radicle t~ bend upwards, until sometimes the point was d1rected to the zenith. vVe must however assume that the after-effects of the irritati~n of the ;ip by an attached object come into play, only after movement has been excited. The tips of the rauiclos of the pea seem to be more sensitive to contact than those of the bean, for when they were extenclod horizontally with squ~res of card adhering to their lower sides, a most curiOus struggle occasionally arose, sometimes ont-:J and sometimes the other force provailino· but ulti- 1 b' 1m ate .Y geotropism was always victorious '· neverthe-ess, m two instances the terminal part became so much curved upwards that loops were subsequently formed. With the pea, therefore, the irritation from an attached object, and from geotropism when actino· ~ . h b ng t angles to the radicle, arc nearly balanced ~or~es. Closely similar results were observed with the onzontally extended radicles of Cucurbita ovifera, 0 2 |
