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Show 19-1 . SUMMARY OF CIIAP'l'Ell. CHAP. III. ver.y s1 I.g ht, a nd the ad. J'oinin. g upp.e r part .b end.s away from the cauterised pomt, with mme certam.ty m most cases tha n fr· om an obJi ect attached on one side. Here I· t 0 b Vl·O US1 Y I·s not the mere tou.c h, but the e. ffect . d d by the caustic, which Induces the t1p to pro uce a· . . . ·t . . transnu' t sorn e I'nfluence to the a JOinmg p. cu '. causmg it to bend away. If one side. of. the t1p IS badly injured or killed by the ca~stw, It ce~ses to grow, whilst the opposite side contmues growmg; a_n~ the resu lt I·S thac t the tip itself bends tow·a rds the mdJu ·r ec·l 'd d often becomes completely hooked; an 1t IS s1 e an d' · · t remarkable that in this case the a .JOmmg upper par does not bend. The stimulus is too powerful or the h k too great for the proper influence to be trans· s oc . l l m1' tte d fr·o m the tip. We have stnct y ana o·g ous cases 1 h' h with Drosera, Dionrea and Pinguicula, wltl. w IC plants a too powerful stimulus docs not excite the tentacles to become in curved, or the lo bcs to close, or the margin to be folded in wards. . . With respect to the degree of scnsitr:eness of the a ex to contact under favourable conditwns, we . ~ave s~en that with Vicia f aba a little square of wntmg· paper affixed with shellac sufficed to cause move· ment. as did on one occasion a square of merely clamp~d goldbeat ers' skin, but it acted very. slowly. Short bits of moderately thick bristle (of whiCh mea· surements have been given) affixed with gum-wate~ acted in only three out of eleven trials, and beads 0 dried shellac under ~l0th of a gra·m m· wei·g ht actec1 only twice in nine cases ; so that here we . h~ve nearly reached the minimum of necessary_ .u ntato· tion. The apex, therefore, is much less sensitive pressure than the glands of Drosera, for these. a{e affected by far thinner objects than bits of bn~t. e, and by a very much less wm· g h t tha n 2o1o th of a gBlll· CHAP. III. SUMMARY OF CHAPTER 105 But the most interesting evidence of the delicate sensitiveness of the tip of the raclicle, was afforded. by its power of discriminating bchve n equal-sized squares of card-like and very thin pap r, when these wcr · attached on opposite sides, as was observed. with the radicles of the bean and oak. When radicles of tho bean arc extenclecl horizontally with squares of card attached 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 radiclcs of any of the above-named genera, suspended vertically, the irritation conquered geotropism, which latter power at first acted obliquely on the radicle ; so that thl:l immediate irritation from the attached obj ect, aided by its after-effects, prevailed anu caused the radicle t~ bend upwards, until sometimes tho point was d1rected to the zenith. vVe must, howcv r, assume that the after-effects of the irritation of tho tip by an attached object come into play, only after movement has been excited. The tips of the rauiclcs of the pea seem to be more sensitive to contact than those of th bean, for when they were extendc<l horizontally with squares of card adherin<r to their lower sides a most . b ' cunous struggle occasionally arose, sometimes one and sometimes the other force prevailing, but ultimately geotropism was always victorious· neverthe-less ' in t wo m· s t ances t h e termi·n al part 'b ecame s< > much curved upwards that loops were subsequently formed. With the pea, therefore, the irritation from an a~tached object, anu from geotropism when actintr !t right angles to the radicle, arc noarl y balanced hw rc.e s · 01 ose 1Y s1· m1- 1a r results were observed with the onzontally extended radicles of Cucurbita ovifera, 0 2 |
