54 



EXPERIMENTAL PLANT PHYSIOLOGY. 



horizontal and the tip is in contact with the mercury. Pour in 

 enough water to form a thin layer on top of the mercury. The 

 root will bend downward with such Fig. 49. 



force as to penetrate the mercury. 

 (Fig. 49.) 



Root of seedling penetrating 

 mercury. (Sachs.) 



44. Influence of Gravity. — 



•Gravity acts in a vertical direc- 

 tion and with a force directly 

 proportioned to the mass of the 

 body acted upon. In plants the amount and rapidity of the 

 curvature of an organ, in response to gravity, depends on 

 its stage of development and the angle which its axis forms 

 with the vertical. Progeotropic organs respond most rapidly 



Fig. 50. 



Seedlings on a revolving wheel driven by a clock. (After OeU.) 



