280 



BOTANY 



Kio. '2-2i>. Different stages in the pro. 

 i I-NS of ^eotropic movement. The 

 figures, 1-16, indicate successive 



These considerations determine tin- actual 

 course of the directive movement of geotropum, 

 which, as will be seen from the adjoining figure 

 (Fig. 225), does not consist merely of a simple. 

 continuous curvature. The numbers 1-16 show, 

 diagrammatically, different stages in the geotropic 

 erection of a seedling growing in semi-darkness 

 and placed in a horizontal position (No. 1). The 

 growth in the stem of the seedling is strongest. 

 just below the cotyledons, and gradually decreases 

 towards the base. The curvature begins accord- 

 ingly close to the cotyledons, and proceeds grad- 

 ually down the stem until it reaches the lo\\ei. 

 no longer elongating, jxirtions. Through the 

 downward movement of the curvature, and partly 

 also through the after-effect of the original 

 stimulus, the apical extremity becomes bent out 

 of the perpendicular (Xo. 7 1 , and in this way 

 a curvature in the opposite direction takes place. 

 Thus, under the influence of the stimulus, the 

 stem bends back wards and forwards, until, iinally, 

 the whole growing portion becomes erect and no 

 longer subject to the one-sided action of the 

 geotropic stimulus. 



Positive Geotropism is exhibited in tap-roots, 

 in many aerial roots, and in the leaf-sheaths of 

 the cotyledons of many Monocotyledons which 

 penetrate the earth during germination. All 

 these organs, when placed in any other position, 

 assume a straight downward direction and after- 

 wards maintain it. Formerly, it was believed 

 that this resulted solely from their weight and 

 the pliancy of their tissues. It is now known 

 that this is not the case, and that positively 

 geotropic, like negatively geotropic. movements 

 are possible only through growth. The power 

 of a downward curving root-tip to penetrate 

 mercury (specifically much the heavier... and to 

 overcome the resistant pressure, much greater 

 than its own weight, proves conclusively that 

 positive heliotropism is a manifestation of a vital 

 process. Positive geotropic curvature is due to 

 the fact that THK GUOWTH OK AN UI:<;AN IN 



LENGTH IS PKOMiilKl' <>N THK VITKI: SIDE, 

 AND RETARDED, T-\ IN' MnliK. STRONGLY. ON THK 

 SIDE TURNED TOWARDS THE EARTH. A young 



stages in the geotropic curvature of 



r, llinK K rown in sen.i-darkness: root of the Chestnut, growing vertically, elon- 



at 1, placed horizontally; at 16, ver- gated equally on all sides 20 mm.; when 



tical. For description of intermedi- placed horizontally, it exhibited a growth of 28 



ate stages see text. (Diagrammatic.) mm OQ the upper an(1 of on]y 9 mm . oll t lic 



lower side. Fig. 226 illustrates the way in which the curvature takes place in the 



