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Show 550 SUMMARY AND CnAP. xn. or oblique fissure in the earth, or a lnnorow made by an earth-worm or larva; and it is certain that roots often run down the old burrows of worms. rl,he tip, however, in endeavouring to circnmnntntc, will continually press against the earth on all sitles, and this can hanlly fail to be of tho highest import<tnee to the plant; for we have soon that when little bits of cardlike paper and of very thin paper wore cemented on opposite sides of tho tip, tho whole oTowing part of the radicle was cxcit c1 to bend away from the side bearing the card or more resistino· substance, towards the side bearing the thin paper. vV c may therefore feel almost sure that when the tip encounters a stone or other o bstaclo in the ground, or even earth more compact on one si<le than tho other, the root will bend away as much as it can from the obstacle or the more 1·esisting earth, and will thus follow with unerring skill a line of least resistance. The tip is more sensitive to prolonged contact with an object than to gravitation when this acts obliquely on the radicle, and sometimes even when it acts in the most favourable direction at right angles to the radicle. The tip was excited by an attached bead of she~la~, weighing less than :r~· 0th of a grain (0·33 m.g.); It JS therefore more sousiti ve than tho most clolwate ten· dril, namely, that of Passijl01·a gracilis, which was barely acted on by a bit of wire weighing 0I0th of a grain. ~ut this degree of sensitiveness is as nothing compm·?d w1th that of the glands of Drosera, for these arc oxmtcd b1 particles weighing only 7 7-40 of a grain. r~ho sen~l· tiveness of the tip cannot bo accounted iol' by Lts being covered by a thinner layer of tissu? than ~h~ other parts, for it is protected by the l'Ohttivoly t~10l\ root-cap. It is remarkable that althoug'h the 1-adwle bends away, when one side of the tip is slightly touched CHAP. XII. CONCLUDING REM:AUKS. G51 with caustic, yet if the .·iclo be much cauterised tho injury is too groat, and tho power of tnmsmittinO' some influence to the adjoining parts cau i11g th m t~ b ml, is lost. Other analogous cases arc known to occur. After a radicle has boon clofl ctocl by some obstacle geotropism diro0ts the tip again to grow porpondieu~ larly downwards; but gcotro1Jism is a fc ble power, and here, as Sachs has shown, another intorostinO' adaptive movement comes into play; for rac.liclos t~ a d~s~ance of a few millimet?rs from the tip are sensitive to prolonged contact In such a manner that they bend towards the touching object, instead of from it as occurs when an object touches one side of the tip. Moreover, the curvatl~ro thus cansed is abrupt; the pressed part alone bending. Even slight pressure suffices, such as a bit of card cemented to one side. Therefor~ a radicle, as it. passes over the edge of any obst~cle m the gr?und., will through tho action of geo. tropism press agamst It; and this prcssuro will cause the radicle. to endeavour to bend abruptly over the edge. It will thus recover as quicldy as possible its normal downward cmu·so. Radicles are also sensitive to air which contains more moistur~ on one sicle than the other, and they bend tow~rd~ Its source. It is therefore probable that th~y are m hke manner sensitive to dampness in the so1l. It was ascertained in several cases that this ~ensitiveness resides in the tip, which transmits an ~nfluenc~ .causing the adjoining upper part to bend m oppositiOn to geotropism towards the moist object. We may therefore infer that roots will be deflected fro~ the~r downward course towards any source of mOisture m the soil. . Again, most or all racli ·los are sliCYhtly sensitive to hght, and, according to vViesncr, O'ene~·ally bend a.littl~ |