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Show 132 SENSITIVENESS OF THE APEX CIIAP. III. hereafter see, Sachs discovered that the r~clicle a litt~e above the apex is sensitive, and bonds hke .a tendnl towards the touching object. But when one s1de of the apex is pressed by any object, th~ growing part be~ds away from the object; and this .seems a. beautiful adaptation for avoiding obstac~es In the sml, .and, as we shall see, for following the lmos of least ros1st~nce. l\'Iany organs, when touched, bend in ~ne fixed drrec· tion such as the stamens of Berbons, the lobes of Dio~rea, &c.; and many organs, such as tendrils, whe· ther modified leaves or flower-peduncles, and some few stems, bend towards a touching object.; but no case, we believe, is known of an organ bendmg am.ty from a touching object. Sensitiveness of the Apex of the Radicle of Vicia faba. -Common beans, after being soake<l in water for 24h., were pinned with the hilum downwa.rcls (in the manner followed by Sachs), inside the cork hds of ?lass-vessels, which were half filled with water; tho sides and the cork were well moistened, and light was excluded. As soon as the beans had protruded radiclos, some to a length of less than a tenth of an inch, and others to a leno·th of several tenths, little squares or oblongs ~f card ~ere affixed to the short sloping sides o~ thell' conical tips. The squares therefore ~dhoro<l obhq?cl~ with reference to the longitudinal ax1s of tho radwl.e, and this is a very necessary precaution, for if the blts of card accidentally became displaced, or wore drawn by the viscid matter employed, so as to adher~ paralle: to the side of the radicle, although only a httle W~J above the conical apex, the radicle did not ~enc~ m the peculiar manner which we are here consldenng. Squares of about the :foth of an l.nch (i.e. about It mm.), or oblong b1· ts of nearly the same si·z e, we re found to OIIAP. III. OF THE RADICLE OF TITE BEAN. 133 be the most convenient and effective. We employed at first ordinary thin card, such as visiting cards, or bits of very thin glass, and various other objects; but afterwards sand-paper was chiefly employed, for it was almost as stiff as thin card, and the roughened s1uface favoured its adhesion. At first wo generally used very thick gum-water; and this of course, under the circumstances, never dried in the least; on the contrary, it sometimes seemed to absorb vapour, so that the bits of card became separated by a layer of fluid from the tip. When there was no such absorption and the card was not displaced, it acted well and caused the radicle to bend to the opposite side. I should state that thick gum-water by itself induces no action. In most cases the bits of car<l were touched with an extremely small quantity of a solution of shellac in spirits of wine, which had been left to evaporate until it was thick; it then set hard in a few seconds, and fixed the bits of card well. vVhen small drops of the shellac were placed on the tips without any card, they set into hard little beads, and these acted like any other hard object, causing the radicles to bend to the opposite side. Even extremely minute beads of the shellac occasionally acted in a slight degree, as will hereafter be described. But that it was the cards which chiefly acted in our many trials, was proved by coating one side of the tip with a little bit of goldbeaters' skin (which by itself hardly acts), and then fixing a bit of card to the skin with shellac which never came into contact with the radicle : nevertheless the radicle bent away from the attached card in the ordinary manner. Some preliminary trials were made, presently to be described, by which the proper temperature was determined, and then the following experiments were made. It should be premised that the beans were |