ALTERATIONS IN GEOTROPIC PROPERTIES 8/ 



find torsions of the stem. 1 Observe the movements of all of the 

 climbing plants to which access can be had and note direction in 

 which the revolving movement is made and time necessary for 

 one revolution. 



134. Behavior of Twining Plants when Freed from the Influence 

 of Gravity. Select a small specimen of one of the species used 

 in 133, not more than 25 cm. in length, growing in a suitable pot, 

 and tie the stem to a stake as before. Now fasten the pot to a 

 clinostat and revolve it in a horizontal position on its long axis. 

 The tip of the shoot should be directed parallel to the surface of 

 a window, and the revolutions of the instrument should be made 

 in less than ten minutes. Note that the stem ceases its revolving 

 motion and grows in a more or less direct line, when freed from 

 the unequal lateral stimulation of gravity (Fig. 39). 



135. Alterations in Geotropic Properties. Vegetative organs 

 generally retain their characteristic geotropic properties during 

 their entire existence unchanged. In some instances however, 

 the form of reaction exhibited may be reversed by the influence 

 of external agents, such as oxygen, or the lack of a proper sup- 

 ply of this element, or by the influence of the phototropic prop- 

 erties of the plant. The reproductive shoots of the higher 

 forms may exhibit direct alteration of geotropic properties cor- 

 responding to the changed needs of the organism in accomplish- 

 ing pollination, seed-distribution, and protection from climatic 

 elements. Thus the buds and flowers of Papaver, flowers and 

 fruits of Aquilegia, Delphinium, and Aconitum change their geo- 

 tropic reactions during the course of pollination, fertilization or 

 development of the seeds (See carpotropism). 2 



136. Recovery from a Position Assumed Geotropically. If a 

 root or shoot has formed a curvature in response to a geotropic 

 stimulus, and is then set in such position that it will be stimulated 

 in a reverse manner the curved portion will first be straightened, 



'Arthur. Exp. in Veg. Physiol. 1897. 



Darwin, C. Climbing Plants. 1876. 



2 V6chting. Bewegungen der Bliiten und Friichte. 1882. 



