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Show 150 SENSITIVENESS OF THE APEX OnAP. III. ture followed. Secondly, thin slices were cut with a razor obliquely off the tips of three radicles similarly suspended ; and after 44 h. two were ~ound plainly bent from the sliced surface; and the thud, the whole apex of which had been cut off obliquely by accident, was curled upwards over the bean, but it was not clearly ascertained whether the curvature had been at :first directed from the cut surface. These results led us to pursue the experiment, an.d 18 radi?les, which had grown vertically downwards 1n damp au, had one side of their conical tips sliced off with a razor. ':rhc tips were allowed just to enter the water in the jars, and they were exposed to a temperature 14°-16° C. (57°-61° F.). The observation~ wore made at ~ifferent times. Three were exammed 12 h. after bemg sliced, and were all slightly curved from the cut suTface; and tho curvature increased considerably after an additional 12 h. Eight were examined after 19 h.; four after 22 h. 30 m. ; and three after 25 h. The :final result was that out of the 18 radiclos thus tried, 13 were plainly bent from the cut surface after the above intervals of time; and one other became so after an additional interval of 13 h. 30 m. So that only 4 out of the 18 radicles were not acted on. To these 18 oases the 3 previously mentioned ones should be added. It may, therefore, be .concluded that a t~in slice removed by a razor from one side of the comcal apex of the radicle causes irritation, like that f:o:n an attached object, and induces curvature from the mJured surface. Lastly, dry caustic (nitrate of silver) was employed to irritate one side of the apex. If one side of the apex or of the whole terminal growing part of a radicle, is by any means killed or badly injured, the other side continues to grow ; and this causes the part CnAP. III. OF THE RADICLE OF 'l'IIE BRAN. 151 to bend over towards the injnrc<l siclc.* But in the following experiments we endeavour d, generally with success, to irritate the tips on oue side, without badly injuring them. r:rhis was effected by first drying the tip as far as possible with blotting-paper, though it ~till remained somewhat damp, and then touching it once with quite dry caustic. Seventeen r<tclicles wore thus treated, and were suspend d in moist air over water at a temperature of 58.) F. They were examined after an interval of 21 h. or 24 h. 'rhe tips of two were found blackened equally all round, so that they rould tell nothing and were rejected, 15 being l ft. Of these, 10 wore curved from the side which had bern touched, where there was a minut brown or blaeki h mark. Five of these racliclcs, three of which wore already slightly deflected, were allowed to ent r the water in the jar, and were re-examined after an additio~ al interval of 27 h. (i.e. in 48 h. after the applicatiOn of the caustic), and now four of them had become hooked, being bent from the discoloured side with their points directed to the zenith ; the fifth remained unaffected and straight. 'rhus 11 radicle out of the 15 were acted on. But tho curvatnre of the four just described was so plain, that they alone would have sufficed to show that the radicles of the bean bend away from that side of the apex which ha~ been slightly irritated by caustic. The power of an I1·ritant on the apex of the Radicle • Oiesielski found this to be tho ~be ('Untersuchungen tiber die 1 wartskriimmung der Wurzel,' hS7l, p. 28~ after l?urning with r ea~ed plattnum one side of a ll~lCle. So did we when we pahnted longitudinally hn.lf of the w ole length of 7 rnclicles, sus-ponded over water, with o. thick !nrer. of greaso, which is very lDJUrwus or oven fatal to growing parts; for after 48 hours five of these rndicles were curved towards tr.e greased sicle, two remaining straight. |
