198 



PLANT LIFE. 



successive moment (fig. 194). If the wheel upon which 

 such seedlings are grown be rotated at a high speed, the cen- 



Fig. 194.— Seedling mustard plants grown on a cube of peat, T, attached to the slowly 

 rotating axle, A, A, of a clinostat. The direction of growth of roots and stems is 

 controlled only by the nearness of moist surfaces, the action of gravity and light being 

 eliminated. Note the variable direction of roots and stems. At m and m^ aerial 

 hyphas of a mold have taken direction as far from the repellant moist surfaces as pos- 

 sible. One half natural size. — After Sachs. 



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 stem will be controlled by a com- 

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

 latter predominating if the speed is great), and will grow in- 

 ward and upward, while the roots will grow downward and 

 outward (fig. 195). 



288. Transverse geotropism. — Not all stems, however, 

 are negatively geotropic, nor all roots positively geotropic. 



