OCR Text |
Show 528 SE~SITIVENESS '1'0 GUAVITATION. C'nAr. XI. case that when tho tips are amputated after an exposure to ..,.00_ tropism of only 45 m., though a slight influence is someti~es transmitted to the adjoining part of the radicle, yet this seldom suffices, and then only slowly, to induce even moderately wollpronouncorl curvature. In tho previously given experiments on 29 horizontally extended raclicles witll their tips amputated, only one grew irregularly in any marked manner, and this became bowed upwards at an auglu of 65°. In Ciesielski's experiments tho raclicles could not have grown very irregularly, for if they hacl dono so, he could not hnvo spoken confl.d.ently of the obliteration of all geotropic act ion. It is therefore remarkable that Sachs, who experimented on many radicles with their tips amputated, found extremely disord.eretl growth to be the usual result. As horizontally extended ra.cliclos with amoutated. tips are some· times acted on slightly by geotropism within a short time, and arc often acted on plainly after one or two days, we thought that this influence might possibly prevent disordered growth, though it wtts not ahle to inclnco immediate curvature. Therefore 13 radicleA, of which 6 hacl their tips amputated tmnsvor::; ely for a length of 1·5 mm., and the other 7 for a length of only 0·5 mm., wore Rusvond d vertically in damp air, in whiclt po. ition they wonlcl not be affected by geotropism; but they exhibited no grcu.t irregularity of growth, whilst observed dming 4 to 6 days. vVo next thought that if care were not taken in cntting oft' tho tips transvorf':ely, one side of the stump might be initatod more than the other, either at first or subsoqnontly during tho regeneration of tho tip, and that thiH might canso tho radicle to bond to one side. It has also been shown in Chapter III. t1mt if a thin slice be cut off one Bide of the tip of the radiele, thiR rn.uses tho radicle to bond from tho sliced side. Accordingly, 30 radicloc;, with tips amputatcrl for a length of 1·5 mm., wore allowed to grow perpendicularly downwards into water. Twenty of them were amputated at an anglo of 20J with a line transverse to their longitudinal axes; aml such stumps appeared only moderately oblique. The romttining ton mdicles wore amputated at an angle of about 45°. Under those l"!ircnmstancos no loss than 19 out of the 30 hocitmc much distorted in tho course of 2 or 3 days. Eleven other mdicles were similn.rly treated, excepting that only 1 mm. (including in this and all other cases tho root-cap) was ampntato<. l; aud of those only one grew much, and two others slightly CRAP. XI. TRANSMIT'l'ED EFFEC'rS : VICIA. [)2!) clistorted; so that this amount of oblique amputation was not sufficient. Out of the above 30 raclicles, only one or two Rhowod in the first 24 h. any distortion, but this became plain in the 19 cases on the second day, and still more conspicuous at the close of the third day, by which time new tips had been partially or completely regenerated. ·when therefore a now tip is r eformed on an oblique stump, it probably i developed sooner on one side than on the other: and this in some manner excites the adjoining part to bend to one side. Hence it seems probable that Sachs unintentionally amputated the radicles on which he experirnented, not strictly in a transverse direction. This explanation of the occasional irregular growth of mdicles with amputated tips, is supported by tho rei:iults of cauterising theirtips; for often a greater length on one side than on the: other was unavoidably injured or killed. It should be remarked that in the following trials the tip · were first dried with blotting-paper, and then slightly rubbed with a dry stick of nitrate of silver or lunar caustic. A few toucl1os with the caustic suffice to kill the root-cap and some of the upper layers of cells of the vegetative point. Twenty-seven radicles, some young and very short, others of moderate length, wore suspended vertically over water, after being thus cauterised. Of these some entered the water immediately, and othcr8 on the secon l day. The same number of uncautcrised radiclos of the same ago were observed as controls. After an interval of throe or four days the contrast in appeamnce between the cauterised and control specimens was wonderfully great. The controls had grown straight downward~, with the exception of the normal curvature, which we have called Sachs' curvature. Of the 'l1 cauteri~ed radicles, 15 had become extremely distorted; 6 of l~1em grew u~wards and formed hoops, so that their tips sometunes came mto contact with tho bean above; 5 grow out rec_tangularly to one side; only a few of the romainin()' 12 were quite straight, and some of these towards the clo:e of our obse ~at 'I ons be came hooked at their extreme lower ends. ~cles, extended horizontally instead of vertically with their ttps t 0 cau emed , also sometimes grew distorted but n' ot so com-mon! f . ' 1 y, as ar as we could Judge, as those suspended vertically· 10r this occurro d W·i th only 5 out of 19 radicles thus treated. ' Instead of cutting off the tips, as in the first set of experimt endts, we next tried the effects of touching horizontally ex-en ed radI' C1e s WI' th causti.c m. the manner J.U st described. Bu. t 2 1\f |