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Show 156 SENSITIVENESS OF THE CHAP. III. inch square, or rather less) were attached in the same manner to one side of the radicle at a distance of 3 or 4 mm. above the apex. In our first trial on 15 radicles no effect was produced. In a second trial on the same number, three became abruptly curved (but only one strongly) towards the card within 24 h. From these cases we may infer that the pressure from a bit of card affixed with shellac to one side above the apex, is hardly a sufficient irritant; but that it occasionally causes the radicle to bend like a tendril towards this side. vVe next tried the effect of rubbing several radicles a.t a distance of 4 mm. from the apex for a few seconds with lunar caustic (nitrate of silver); and although the radicles had been wiped dry and the stick of caustic was dry, yet the part rubbed was much injured. and a slight permanent depression was left. In such cases the opposite side continues to grow, and the radicle necessarily becomes bent towards the injured side. But when a point 4 mm. from the apex was momen· tarily touched with dry caustic, it was only faintly discoloured, and no permanent injury was caused. rrhis was shown by several radicles thus treated straighten· ing themselves after one or two days; yet at first they became curved towards the touched side, as if they had been there subjected to slight continued pressure. These cases deserve notice, because when one side of the apex was just touched with caustic, the radicle, as we have seen, curved itself in an opposite direction, that is, away from the touched side. The radicle of the common pea at a point a little above the apex is rather more sensitive to continued pressure than that of the bean, and bends towards the pressed side.* We experimented on a variety (York· * Sachs, 'Arbeiten Bot. Institut., Wiirzburg,' Heft iii. p. 438. CHAP. III. UPPER PART OF THE RADICLE. 157 shire Hero) which has a much wrinkled touo-h skin b ' too large for the included cotyledons; so that out of 30 peas which had been soaked for 24 h. and allowed to germinate on damp sand, the radicles of three were unable to escape, and were crumpled up in a strano-e manner within the skin; four other radicles we~·e abruptly bent round the edges of the ruptured skin against which they had pressed. Such abnormalities would probably never, or very rarely, occur with forms developed in a state of nature and subjected to natural selection. One of the four radicles just mentioned in doubl~ng backwards came into contact with the pin by whiCh the pea was fixed to the cork-lid ; and now it bent at right angles round the pin, in a direction quite different from that of the first curvature due to contact ~ith th~ ruptured skin; and it thus afforded a good 1llustrat10n of the tendril-like sensitiveness of the radicle a little above the apex. Little squares of the card-like paper were next ~ffixed to radicles of the pea at 4 mm. above the apex, m ~he same manner as with the bean. Twenty-eight radicles suspended vertically over water were thus treated on different occasions, and 13 of them became curved towards the cards. The greatest degree of curvature amounted to 62° from the perpendicular; but s? large . an angle was only once formed. On one ~cas10n a shght curvature was p rceptible after 5 h. 5 m., and it was generally well-marked after 14 h. ;~ere. can therefore be no doubt that with the pea, rn~at10n from a bit of card attached to one side of the radicle above the apex suffices to induce curvature. Square~ of card were attached to one side of the tips of. 11 rad1eles within the same jars in which the above tnadls were made, and five of them became plainly an on e sl 1' g h tly, curved away from this side. Other' |