174 



OUTLINES OF PLANT LIFE. 



trifugal force will become a constant one, and, acting in 

 place of the neutralized force of gravitation, will determine 

 the direction which the stems and roots will assume. Since 

 the primary stems of most plants are negatively geotropic, 

 when grown under such conditions they will turn toward the 

 center of the wheel, while the positively geotropic roots grow 

 toward the rim. Similarly, if the wheel be rotated rapidly 

 in a horizontal plane the parts will be controlled by a com- 

 bination of the force of gravity and the centrifugal force (the 

 latter predominating if the speed is great) ; the stem will 

 grow inward and upward, while the roots will grow down- 

 ward and outward (fig. 128). 



Fig. 128. — Part of centrifuge, a, the axle, rotated at a high speed by water or electric 

 motor, to which is attached the circular metal piate, r, r, carrying a disk of cork, k. 

 To the latter are attached two seedling beans, W , 5, by means of pins ; st, the primary 

 stem ; A, the primary root. Ove'- the seedlings the cover, a, is placed to keep them 

 moist. After a few hours the lateral roots have turned into the direction of the cen- 

 trifugal force, which was sufficiently powerful to overcome that of gravity except near 

 axis of rotation, x. One halt natural size.- -After Sachs. 



EXERCISE XLI. 



To show the effect of gravity as a stimulus on roots. 



Arrange the marked root of a seedling bean as in Tf 205, except that 

 the root is horizontal, and a pin just above the extremity marks its posi- 

 tion. After 24 hours observe curvature and which spaces have become 

 curved. Compare vi^ith those which have grown most. 



To show the effect of gravity as a stimulus on growing regions of upright 

 leaves. 



