88 RELATION OF PLANTS TO GRAVITATION 



and then recurved in the opposite direction. If however, the first 

 position has been held for a length of time sufficient to permit of 

 any notable growth or development of the tissues, the curvature 

 will become fixed and permanent. The second curvature is gen- 

 erally slower than the first. Roots which need but two or three 

 hours to attain a position of equilibrium in response to the first 

 stimulation, will require 1 2 to 24 for the unbending and recurving 

 in a new position. The above statement applies only to growing 

 organs and not to pulvinar movements. 



137. Recovery of Curvature of Roots. Place a root of pea or 

 bean in an inverted position by fastening the seedling to a sheet 

 of cork in a moist chamber for two or three hours, or until a full 

 curvature has been accomplished. Now alter the position of the 

 organ until the tip again points upward, so that it will reverse or 

 straighten the curvature in order to reach a position of equilibrium 

 with respect to gravity. Note time and manner of this second 

 reaction. 



138. Formative Influence of Gravity. Cut two willow, or pop- 

 lar branches 25 cm. in length, and place in a moist chamber the 

 atmosphere in which is kept completely saturated. Suspend one 

 twig in a horizontal position, and the other in a vertical position 

 with the apical end downward. The polarity of the twigs would 

 induce them to form leaves from the apical portions of both twigs, 

 and roots from the basal end. In response to this, such forma- 

 tion will occur at the beginning of the experiment. If the test is 

 continued for a few weeks however, it will be seen that roots 

 continue to develop from the lower side or end of the shoots, and 

 leaves from the upper sides or ends (See correlations). 



The above test may be performed most successfully if the 

 twigs are taken in the spring immediately before breaking out of 

 the buds. The moist chamber should be placed in diffuse light 

 which should come from several directions, or the experiment 

 may be carried on in the dark chamber. 1 



1 Vochting, H. Ueber Organbilding im Pflanzenreich. 1884. 



