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Show 566 SUMM:ARY AND CHAP. XII. the cotyledons of course remained expanded ; both pots were now placed close together in the middle of the room, and the cotyledons which had been exposed to the sun, immediately began to close, while the others opened; so that the cotyledons in tho two pots moved in exactly opposite directions whilst exposed to tho same degree of light. vV e founu that if seedlings, kept in a dark place, wore laterally illuminated by a smttll wax taper for only two or three minutes at intervals of about threequarters of an hour, they all became bowed to the point where the taper had been held. We felt much surpri d at this fact, and until we had read 'Wiesner's observations, we attributed it to the after-effects of the light; but he has shown that the same degree of curvature in a plant may be induced in the eourse of an hour by several interrupted illuminations lasting altogether for 20 m., as by a continuous illumination of 60 m. vV e believe that this case, as well as our own, may be explained by the excitement from light b ing due not so much to its actual amount, as to the difference in amount from that previously receivou; and in our case there were repeated alternations from complete darkness to light. In this, and in several of the above specified respec~s, light seems to act on the tissues of plants, almost m the same manner as it does on the nervous system of animals. There is a much more strikinO' analoO'y of the same b b . kind, in the sensitiveness to light being localised m the tips of the cotyledons of Phalaris and A ve?a, and in the upper part of the hypocotyls of Brasswa and Beta· and in the transmission of some infiuence from these' upper to the lower parts, causing the latter to bend towards the light. This influence is also trans· 0JUP, XII. CONCLUDING REMARKS. 567 mitted beneath the soil to a depth 1 . w 1ere no ho·ht enters. It follows from this localisat' th _· b f h 1011, at the lower parts o t e coty lodons of Phalaris & 1 · ·] 11 b b ' c., w 11 l norma y ecome more ent towards a 1 t 1 1 . a era 1g 11 t than. the upper parts, ma. y be briO'htly 1'1 1 umm· ate d. durmg many hours, anu Wlll not Lend in th 1 t . f all light be excluded from the tip. It is a e. eta~ 't1 . 1 n m eros .- ing expenment to p ace caps over the tips of the cotyledons of Phalar~s, and t~ allow a very little liO'ht to enter through m1nuto onf;icos on one side of tho caps, for the lower part of tho cotyledons will th bend to t h1. s SI' de , an d not to the side which has bee n brightly I'l lu m·m ate d du n·n g the whole time. In theen case of the radicles of Sinapis alba, sensitiveness to light also resides in the tip, which, when laterally illuminated, causes the adjoining part of the root to bend apheliotropically. Gravitation excites plants to bend away from the centre of the earth, or towards it, or to pia e themselves in ~ ~ra~sverse. position with respect to it. Although It IS Impossible to modify in any direct manner the attraction of gravity, yet its influence could be moderated indirectly, in the several ways described in the tenth chapter ; and under such ~ircumstances the sa~e kin? of evidence as that given m the chapter on HehotTopism, showe lin the plain st ~nner th~t apogeotropic and geotropic, and probably d1ageotrop1e movements, are all modified forms of circumnutation. D~ft'erent parts of the same plant and different speCies are affected by gravitation in widely different degrees and manners. Some plants and organs exhibit hardly a trace of its action. YounbO' seecllinO's which a.s we kn ow, m. rcumnutate rapidly, are eminenot ly sensi-' tive; and we have seen the hypocotyl of Beta bending |