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Show 180 SENSITIVENESS OF THE APEX 0HA1. III. seed, but not laterally from _the prim~ry one. ~en of these secondary radicles, whiCh were du:ccted obliquely downwards, were experimented on w1th very small squares of card attached with shellac to the lower sides of their tips. If therefore the s~u~res acted, ~he radicles would bend upwards in oppos1t10n to gravtty. The jar stood (protected from light) on a s~nd-bath, which varied between 76° and 82° F. After only 5 h. one appeared to be a little deflected from the square, and after 20 h. formed a loop. Four others . re considerably curved from the squares after 20 h., "e . h . . and three of them became hooked, 'nth t cu t1ps pointing to the zenith,-one after 29 ~· and. the two others after 44 h. By this latter t1me a s1~th radicle had become bent at a right angle from the s1de bearing the square. Thus altogether six out of ~he ten secondary radicles were acted on, four not bemg affected. There can, therefore, be no dou 1t that. the tips of these secondary radiclcs arc sensitive to shgbt contact and that when thus excited they cause the upper ~1art to bend from t~e touching o~jcct; b~t genera1ly, as it appears, not 11~ so short c"L tm10 as m the case of the first-formed rathcle. SENSITIVENESS OF 'l'HE TIP OF TilE UAVICL.E '1'0 MoisT AIR. Sachs made the interesting discoYcry, a few years ~go that the raclicles of many seedling plants bend tow'a rds an a dJ. O.m .m g d amp sm· £a ce. * We shall . h.e re endeavour to show that this peculiar form of_ sen.sltive· ness resides in their tips. rrhe movement JS d:rectJy the reverse of that excited by the irritants hit.herto considered, which cause the growing part of the * 'Arbciten des Bot. lnstitut., 1· n un •"u rzb urg, 'vol · i · 1872 ' P· , 209· CHAP. III. OF THE RADICLE TO l\IOIST AIR. 1~1 radicle to bend away from tho source of irritation. In our experiments we followed Sachs' plan, and sieves with seeds germinating 1n damp sawdust were suspended so that the bottom was generally inclined at 40° with the horizon. If the radicles had been acted on solely by geotropi8m, they would have grown out of the bottom of the sieve perpendicularly downwards; but as they were attracted by the adjoining damp surface they bent towards it and were deflected 50° from the perpendicular. For the sake of ascertaining whether the tip or the whole growing part of the radicle was sensitive to the moist air, a length of from 1 to 2 mm. was coated in a certain number of cases with a mixture of olive-oil and lamp-black. This mixture was made in order to give consistence to the oil, so that a thick layer could be applied, which would exclude, at least to a large extent, the moist air, and would be easily visible. A greater number of experiments than those which were actually tried would have been necessary, had not it been clearly established that the tip of the radicle is the part which is sensitive to various other irritants. Phaseolus multi)lo1·us.-Twenty-nine radiclcs, to which nothing had been done, growing out of a sieve, were observed at the same time with those which had their tips grea. ed, and for an equal length of time. Of the 29, 24 curved themselves so as to come jnto close contact with the bottom of the sieve. The place of chief curvature was generally at a distance of 5 or 6 mm. from the apex. Eight radicles had their tips greased for a length of 2 mm., and two others for a length of ~~ mm,; they were kept at a temperature of 15°-16° C. After Intervals of from ] 9 h. to 24 h. all were still vertically or almost vertically dependent, for some of them had moved towards the adjoining damp surface by about 10°. They had therefore not been acted on, or only slightly acted on, by the damper air on one side, although the whole upper part was freely exposed. After 48 h. three of these radicles became |