SECT. II 



PHYSIOLOGY 



257 



Positive Geotropism, on the other 

 hand, is observable 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 «pV^ 

 position,' assume a straight downward 

 direction and afterwards maintain it. 

 Formerly, it was believed that this re- 

 sulted solely from their weight and the 

 pliancy of their tissues. It is now known 

 that this is not the case, and that posi- 

 tive 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 con- 

 clusively that positive heliotropism is a 

 manifestation of a vital process. Posi- 

 tive geotropic curvature is due to the fact 



that THE GROWTH OF AN ORGAN IN 

 LENGTH IS PROMOTED ON THE UPPER SIDE, 

 AND RETARDED, EVEN MORE STRONGLY, 

 ON THE SIDE TURNED TOWARDS THE 



earth. A young germinal root of Vicia 

 Fuba, growing vertically, elongated equally 

 on all sides 24 mm.; when placed hori- 

 zontally, it exhibited a growth of 28 mm. 

 on the upper and of only 15 mm. on the 

 lower side. A root of Castanea n'sca, 



with a growth in a vertical direction of 

 20 mm., showed, in a horizontal position, 

 a growth of the upper side of 28 mm. 

 and of the under of only 9 mm. In 

 these experiments, by marking with 

 Indian ink, the unequal elongation in the 

 downward curvature may be demon- 

 strated by the greater divergence of the 

 marks on the upper than the lower 

 side ; it is also evident that, as in 

 negative heliotropism, the curvature 

 takes place in the region of greatest 

 elongation (Fig. 203). As the portion 

 of a root capable of elongation is very short, no excessive over 



s 



Fig. 202. — Different stages in the pro- 

 cess of geotropic movement. The 

 figures, 1 - 10, indicate successive 

 stages in the geotropic curvature of 

 a seedling grown in seini-darkness ; 

 at 1, placed horizontally ; at 16, ver- 

 tical. For description of intermedi- 

 ate stages, see text. (Diagrammatic.) 



