198 



PLANT LIFE. 



successive moment (fig. 194). If tlie wfieel upon which 

 such seedlings are grown l)e rotated at a high speed, the cen- 



FlG. 194. — St!i!d!ing mustard plants grown on a eube of jieat, T^ attached to the slowly 

 rotating axle, -^, .-J, of a clinostat. Tiie 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 "; and ///,; aerial 

 Iiyph."e 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 Ijecome a constant one, and, acting in 

 jilace 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 negativel)- geotropic, 

 when grown under such conditions they will turn toward the 

 center of the wheel, while the positively geotro]jic 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 f)f the force of gra\-ity and the centrifugal Ibrce (the 

 latter [jredominating 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. 



