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Show 142 SENSITIVENESS OF TITE APEX CuAr. III. other trial, for it occurred to us that sensitiveness is easily affected by external conditions, and that radicles growing naturally in the earth in the early spring would not be subjected to a temperature nearly so high as 70° F. We therefore allowed tho racliclos of 12 beans to grow at a temperature of between 55° and (30° F. The result was that in every one of these cases (included in tho abovo-<loscribo<l experiments) the radicle was deflected in tho course of a few hours from the attached object. All the above recorded successful trials, and some others presently to be given, were made in a sitting-room at tho temperatures just specified.. It therefore appears that a temperature of about, or rather above, 70° F. destroys the sensitiveness of the radicles, either directly, or indirectly through abnormally accelerated growth; and this curious fact probably explains why Sachs, who expressly states that his beans were kept at a high temperature, failed to detect the sonsiti voncss of the apex of the radicle. But other causes interfere with this sensibility. Eighteen rac.liclcs were tri ed with little squares of sandeJ ·ar<l, some affixed with shellac and some with gum-water, during the few last days of 1878, and. few first days of the next year. They were kept in a room at the proper temperature during the day, but were probably too cold. at night, as there was a hard frost at the time. The m<licles looked healthy but grew very slowly. 'rhe result was that only 6 out of the 18 were deflected from the attached. car<ls, and this only to a sliaht debaree and at a very slow rate. These b . h radiclos therefore presented. a striking contrast w1t the 44 above described.. On March Gth and 7th, when the temperature of the room varied between 53° and 59° F., eleven germinating beans were tried in the OrrAP. III. OF TITE RADICLE OF TITE BEAN. 143 same manner, and now every one of the radicles became curved away from the canis, though one was only slightly deflected. Some horticulturists believe that certain kinds of seeds will not g rminato properly in the middle of the winter, althouo·h k pt at a right temperature. If there really i · any proper period for the germination of the l> an, th fc Ll d crrce of sensibility of the above radicl os may ha r~ ultod from the trial having been ma<lc in th mid<lle of tho winter, and not simply from th nights being too cold. Lastly, the radicles of four beans, whieh from some innate cause gorminat d later than all tho others of the same lot, m:d .which ~Pw slowly though appearing healthy, were similarly tnc<l, and even aft r 24 h. they were hardly at all deilectocl from th atta hod cards. We may therefore infer that any cau. c whieh r ndcrs the growth of tho radi ·les either slower or mor rapid than t?e ?ormal rate, 1 sons or annul the on ibility o! theu tips to contact. It deserves particu1<:1.r attentiOn that when tho attached objects fnile<l to act there was no bending of any kind, excoptincr Sachs' 'curvature. The force of our evidence would have been gre~tly weakened if occasionally, though rarely, the radwles had become curved in any direction indopendently of the attachocl objects. In the fo regoing numbered paragraphs, however, it may be obs •rvod ~at the extreme tip sometimes b com s after a con- Slderabl · t 1 f · ' b . em erva o tim , abruptly curv d toward tho 1t o.f card·, b u t thI' s I·s a t ota. lly a·I St·i nct phcnom non, as Will presently be explained. Summar¥ of the Results of the foregoing Er:rpe1·iments (bthe Radtcles of Vioia faba.-Altogothor little squares ~ o~t nth of an inch), generally of sanded paper th . sktrff as thin card (between ·15 and ·20 rum. in 1c ness) 'someti·m es of ord·m ary card, or little frag- 11.1·• 1::::1 /, |