io8 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



PART V 



THE INFLUENCE OF GRAVITY AND OF CENTRIFUGAL FORCE UPON GROWTH 



SECTION 29. 



Since all parts of the plant are subject to the laws of mass-attraction, a 

 sufficient increase of the force of gravity would render plants unable to bear 

 their own weight. A similar effect is produced on seedlings of Lupinus luteus 

 when subjected to a centrifugal force 30 times greater than that of gravity 1 , 

 and if it were increased to 1,000 g. the stoutest stem would probably be 

 unable to remain erect. Such forces cause abnormal displacements of the 

 contents of large cells 2 , and these are sufficient to cause the death of a 

 filament of Chara when subjected for a short time to a centrifugal force 

 equal to 2,000 g. 3 In fact the application of a sufficiently powerful 

 centrifugal force must ultimately render the development of every plant 

 impossible. 



The trifling differences in the force of gravitation at different parts 

 of the earth's surface exercise no physiological effects worth mentioning, 

 and in its constant and continuous action gravity differs markedly from 

 such variable factors as light, heat, and the supply of nutriment. These 

 agencies may act either as diffuse or as unilateral stimuli, whereas gravity 

 always acts perpendicularly to the surface of the earth 4 . The weight 

 and statical moment of an organ may be altered by growth, and hence 

 also the mechanical demands it makes upon the supporting parts. In this 

 way stimulatory reactions due to pressure and tension may be aroused. 



Most plants have acquired by adaptive modification the power 

 of responding to gravity by setting themselves in a definite position, and 

 among such irritable responses the geotropic curvatures of roots are most 

 prominent. Gravitation is also responsible for various barymorphotic 

 reactions, including certain forms of dorsiventrality and of polarity. In 



1 Schwarz, Unters. a. d. Bot. Inst. zu Tubingen, 1881, Bd. I, p. 80. [The plants will 

 naturally be fixed with the free portion of the stem pointing inwards to the axis of rotation, and the 

 required force would be obtained at the rim of a wheel of i foot radius when performing 30 rotations 

 per second. The centrifugal force at the earth's surface, due to its rotation, partly counteracts the 

 force of gravitation, but only to a very slight extent, since even at the equator it only corresponds 

 to an acceleration of 0-12 ft. per sec. 2 , or about ^-^th that of gravity. Thus if v = velocity, r = 

 radius, a = acceleration, and T= time of one rotation, 



then a = ; but v = .*. a *= - ^- = 0-12 ft. per sec. 2 



Hence the force of gravity at the earth's equator would increase hardly at all if the earth ceased to 

 rotate, and it would only be neutralized when the earth performed a revolution every i hours.] 



2 [Owing to the varying densities of the cell-constituents. If they all had the same density no 

 .such displacement would occur.] 



3 Mottier, Annals of Botany, 1899, Vol. xm, p. 346. 



* [Nevertheless as regards the plant it is a more perfectly diffused stimulus than any other, since 

 it has practically exactly the same intensity at all points of any growing organ.] 



