THE MOVEMENTS OF PLANTS. 



173 



weight of the parts concerned. Such effects are to be seen 

 in the downward bending of some plants with slender 

 branches, or the curvature of the flower or fruit stalks by the 

 weight of the parts. True geotropic curvatures are brought 

 about by acceleration of the growth of the irritable cells, and 

 the curvatures produced may even be contrary to the direc- 

 tion of the force. If seedlings be grown in boxes upon the 

 rim of a wheel rotating slowly in a vertical plane, so that 

 they are successively subjected to the action of gravity in 

 relatively different directions, it vifill be seen that while their 



Fig. 127. — Seedling mustard plants grown on a cube of peat, 7', 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 w/ and w/j aerial 

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

 sible. One half natural size.— After Sachs. 



members grow in nearly straight lines, the direction assumed 

 by the stems and roots is quite as frequently abnormal as 

 normal, because the effect of gravity which normally deter- 

 mines the direction of growth of these axes is neutralized, 

 since it now acts upon them from a new direction at each 

 successive moment (fig. 127). If the wheel upon which 

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



