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Show ~192 CONCLUDING REMARKS. CHAP. IX. injurious. A. species ,,·hich has ceased to be heliotropic might also be rendered apheliotropic by the preservation of the individuals which tended to circumnutate (though the cause of this and most other variations is unknmvn) in a direction moro or less opposed to that whence the light proceeded. In like manner a plant might be rendered diaheliotropic. CHAP. X. MOVEMENTS EXCITED BY GRAVI'rATION. 493 CHAPTER X. MonrFIED CnwuMNUTATION: MovEMENTs EXOtTED BY GnAvtTA'riON. Means of observation -Apogeotropism- Cytisus-V erhena-BctaGradual eonverr:.ion of the movcmPnt of eircumnutation into apogeetropism in Rubus, Lilium, Phalaris, Avena, and Bra,sica-Apogeotropism retarded by h~liotropism-Efl"ected by the aid of juiuts or pulvini-Movements of flower-peduncl es of Oxalis-Goneral remarks on apogeotropism-Geotropisru-Movcments of rarliolesBurying of seed-capsules-Use of proccss-'rrifolium subterraneum -Arachis-Amphicarpma-Diu geotropism-Cone! usion. OuR object in the present chapter is to show that geotropism, apogeotropism, an<l diageotropism are modified forms of circumnutation. Extremely fine filaments of glass, bearing two minute triangles of paper, were fixed to the summits of young stems, frequently to the hypocotyls of seedlings, to flower-peduncles, radicles, &c., and the movements of the parts were then tr~ced in the manner already described on vertical and horizontal glass-plates. It should be remembered that as the stems or other parts become more and more oblique with respect to the glasses, the figures traced on them nocessaril y become more and more magnified. The plants were protected from light, excepting whilst each observation was being made, and then the light, which was always a dim one, was allowed to enter so as to interfere as little as possible with the movement in progress ; and we did not detect any evidence of such interference. When observing the gradations between circumnu- |
