INFLUENCE OF GRAVITY AND OF CENTRIFUGAL FORCE 109 



this respect, however, gravity is not as manifold in its action as is light, 

 nor is it of such fundamental importance as Hofmeister supposed \ From 

 a phylogenetic point of view, however, the ever-present and approximately 

 constant force of gravity probably played an important part in the 

 induction of fixed polarity, which even now is capable of modification 

 by the same force. 



The inherent tendency to form buds at the apical end of the axis, and roots 

 at its basal end, is influenced or even suppressed by various external conditions, 

 including gravity. Thus when the downwardly growing rhizomes of Yucca or 

 Cordyline are inverted, the hitherto dormant apical bud begins to develop 2 . 



In many plants gravity favours the development of buds upon the upper 

 surface of twigs laid horizontally, especially at the apical end, and distinctly influences 

 or induces the development of roots from the under surface of the basal end 3 . 

 Hence in horizontal or obliquely inclined shoots of Opuntia Ficus-indica *, as well 

 as in the root-tubers of Thladiantha dubia 5 , the buds develop mainly on the upper 

 side, while a renewed formation of roots is induced on the under surface of 

 branches of Heterocentron diversifolium when laid horizontally 6 . Similarly the action 

 of gravity is such as to favour the development of rhizoids on the under surface of 

 gemmae of Marchantia' 1 ', and on that of the female prothallus of Marsilia 8 . 



In a few cases the anisophylly of leafy shoots and of flowers is induced or 

 strengthened by gravity, which is in some cases responsible for the unequal 

 anisotropic thickening of plagiotropic organs 9 . Nordlinger 10 , for example, observed 

 that the concentric (isotrophic) growth in thickness of a vertical stem was replaced 

 by excentric (anisotrophic) growth when it was fixed in a nearly horizontal position. 

 These experiments do not, however, afford conclusive proof, since the influence of 

 the changed tensions and pressures on the upper and under surfaces has not been 



1 Hofmeister, Allgem. Morphol., 1868, p. 579. 



2 Sachs, Arb. d. Bot. Inst. in Wiirzburg, 1880, Bd. II, p. 475; Lectures on Physiology, 1887, 

 P- 536 ; Vochting, Bot. Ztg., 1880, p. 601. A similar result is, however, also produced by cutting 

 off the rhizomes. 



3 Vochting, Organbildung im Pflanzenreich, 1878, I, p. 164; 1884, II, pp. 40, 95 ; Sachs, Arb. 

 d. Bot. Inst. in Wiirzburg, 1880, Bd. II, p. 474. Earlier observations were made by Duhamel, 

 Physique d. arbres, 1758, T. II, p. 122. 



* Sachs, 1. c., 1882, p. 760. 



5 Id., 1. c., p. 704. 



6 Vochting, Organbildung, 1878, I, p. 189. 



7 Pfeffer, Arb. a. d. Bot. Inst. in Wiirzburg, 1871, Bd. I, p. 77 ; Unters. a. d. Bot. Inst. zu 

 Tubingen, 1885, Bd. I, p. 529 (Marckantia) ; Leitgeb, Bot. Ztg., 1872, p. 766 ; Kny, Entwickelung 

 d. Marchantiaceen, 1875, p. 12 (repr. from aus Nova Acta d. Leopold., Bd. xxxvn) (Lunularia). 



8 Leitgeb, Zur Embryologie d. Fame, 1878, p. 7 (repr. from Sitzungsb. d. Wien. Akad., Bd. 

 LXXVII, i. Abth.). 



9 Wiesner, Ber. d. Bot. Ges., 1895, p. 481 ; 1896, p. 181 ; Sitzungsb. d. Wien. Akad., 1892, 

 Bd. ci, I, p. 677 ; Bot. Ztg., 1882, p. 697; Biisgen, Waldbaume, 1897, p. 99; Haberlandt, Physiol. 

 Anat., 2. Aufl., p. 513 ; Detlefsen, Arb. d. Bot. Inst. in Wiirzburg, 1882, Bd. II, p. 686 ; Kny, Bot. 

 Ztg., 1877, p. 417 ; Hofmeister, Allgem. Morphol., 1868, p. 604; Mohl, Bot. Ztg., 1862, p. 274; 

 Schimper, Ber. d. Naturf.-vers. in Gottingen, 1854, P- 87 ; de Candolle, Prlanzenphysiol., 1833, 

 Bd. I, p. 71. 



10 Nordlinger, Der Holzring als Grundlage d. Baumkorpers, 1871, p. 24; Wiesner, Ber. d. Bot. 

 Ges., 1896, p. 1 80. 



