224 PLANT PHYSIOLOOY 



instance, Sachs's familiar cylinder of wire-netting filled witli 

 damp moss or moist earth.* If this cylinder be hung- 

 slanting at an angle of about 45°, the roots will not grow 

 vertically downward away from the cylinder, but will follow 

 along its under side in spite of the geotropic stimulus (p. 

 204). In this experiment the roots bend in the direction 

 from which they obtain water. In the first experiment the 

 absorption of water is followed by expansion, swelling. 

 In the second experiment that side expands which is not 

 able directly to absorb water, while that side which does 

 absorb water does not expand as much. In the first 

 experiment the physical infiuence is uniform on all sides 

 of the root, which swells uniformly. In the second ex- 

 periment, the physical influence was greater on one side 

 than on the other, the tendency to swell on one side 

 was offset by the increased activity of the protoplasm, 

 the more rapid growth, of the opposite side, in response to 

 stimulus. Such a reaction as this is called hydrotropism. 



The hyphae of Mucor grown on moist bread over water 

 are deflected when they reach the surface of the water, grow- 

 ing instead horizontally over it and against the bread. 

 This bending away from water is not negative hydrotro- 

 pism, but rather the reaction of the hyphse to other stim- 

 uli such as food, oxygen, etc., which are more abundant in 

 the bread than in the water. 



Sachs's experiment (see above), with roots deflected by 

 moisture from the vertical position, reveals a very important 

 principle. It is manifestly more important for the root as 

 an absorbing organ to grow where it will obtain needed 

 aqueous solutions than for it to be in any particular posi- 

 Tfcion with relation to the vertical; but in order that the 

 older parts of the root may give the needed mechanical 

 support to the plant, the root must grow downward into 

 the soil. There must therefore be a delicate balance in the 

 sensitiveness and the reactions to geotropic and hydro- 

 tropic stimuli. 



* Sachs, J. von. tJber die Ablenkung der Wurzeln von ihrer normalen 

 Wachsthumsrichtung durch feuchte Korper (Hydrotropismus). Arb. d. 

 bot. Inst. Wurzburg, Bd. I., 1871-4. Gesam. Abhandl., Bd. II., 1893. 



