been increased in amplitude and often changed in direction; and here the stimulus is the alternation of light and darkness, aided, however, by inheritance.  In the case of heliotropism, the stimulus is the unequal illumination of the two sides of the plant, and this determines, as in the foregoing cases, the modification of the circumnutating movement in such a manner that the organ bends to the light.  A plant which has been rendered heliotropic by the above means, might readily lose this tendency, judging from the cases already given, as soon as it became useless or [page 492] injurious.  A species which 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 unknown) in a direction more or less opposed to that whence the light proceeded.  In like manner a plant might be rendered diaheliotropic. [page 493]

CHAPTER X.

Modified circumnutation:  Movements excited by gravitation.

Means of observation — Apogeotropism—­Cytisus—­Verbena—­Beta—­Gradual conversion of the movement of circumnutation into apogeotropism in Rubus, Lilium, Phalaris, Avena, and Brassica—­Apogeotropism retarded by heliotropism—­Effected by the aid of joints or pulvini—­Movements of flower-peduncles of Oxalis—­General remarks on apogeotropism—­Geotropism—­ Movements of radicles—­Burying of seed-capsules—­Use of process—­Trifolium subterraneum—­Arac
his—­Amphicarpaea—­Diageotropism—­Conclusion

Our object in the present chapter is to show that geotropism, apogeotropism, and 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, etc., and the movements of the parts were then traced 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 necessarily 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-[page 494] tation and heliotropism, we had the great advantage of being able to lessen the light; but with geotropism analogous experiments were of course impossible.  We could, however, observe the movements of stems placed at first only a little from the perpendicular, in which case geotropism did not act with nearly so much power, as when the stems were horizontal and at right