62 EXPERIMENTAL PLANT PHYSIOLOGY. 



48. Hydrotropism. — The moisture of the medium which 

 surrounds the plant induces movements in certain organs 

 either toward or away from the source of the moisture. The 

 property of an organ by which it reacts to moisture is termed 

 hydrotropism. By this power roots direct their apices toward 

 portions of the soil containing the proportion of moisture best 

 suited to their specific needs. 



EXPERIMENT 76. 



HYDROTROPISM OF ROOTS. 



Cover a zinc box, 5 cm. wide and 20 cm. long, open on two sides, 

 with gauze after it has been filled with moist sawdust, containing 

 swollen seeds of Bean, Pea, or Fig. 58. 



Corn. Suspend the apparatus 

 under a pasteboard box, so that it 

 hangs at an angle of 45 degrees. 

 After a time the roots issue through 

 the openings in the gauze beneath ; 

 they do not follow geotropism and 

 grow directly downward, however, 

 but press against the layer of moist 

 sawdust. Place the apparatus in a 

 damp chamber where the moisture 

 is equal in all directions from the 

 roots, and they grow directly down- 

 ward in response to the stimu- 

 lus of gravity. In this case the 

 roots receive the same stimulus 

 from moisture in all directions, 

 and in consequence no reaction to 

 it is shown. They are free to re- Hydrotropism of roots. (Detmer.) 

 spond to their progeotropic tendency. 



49. Contact Movements. — Many plants will exhibit move- 

 ments so rapid as to be visible to the naked eye when touched 

 or struck with any hard object. These movements serve vari- 

 ous purposes in different groups of plants. In some instances, 

 as in the Mimosa, this is a device for protecting the leaves 



