84 RELATION OF PLANTS TO GRAVITATION 



ing while those on the upper are convergent. Examine for dif- 

 ferences in structure. 



128. The Geotropic Relations of Dorsiventral Organs. The 

 greater number of dorsiventral organs, such as leaves, are dia- 



geotropic, although many well- 

 known exceptions occur. Such 

 organs are in equilibrium only 

 when the axis subtends a certain 

 fixed angle with the line of grav- 

 ity, and when the two sides oc- 

 'a~ b cupy their proper and relative po- 



Fig. 38. A, stem and branches of sitions. The geotropic reactions of 



a pine tree 7.5 meter, in height from such organsmay show tw0 move . 

 which the top has been removed. B, 



same * year later showing upward ments : a curvature to bring the 



curvature of a branch seven years old. axis toward the horizontal, and a 



ter J ost - twisting or torsion to place the 



surfaces in their normal relative positions. These reactions are 

 generally complicated with phototropic movements in plants 

 growing in the open, and are not capable of easy analysis. 



129. Rotation and Curvature of Petioles of Dorsiventral Leaves 

 in Response to Geotropic Stimuli. Secure a vigorous specimen 

 of any species of Helianthus growing in a pot. Place the axis of 

 the plant in a horizontal position in a dark room and note the 

 positions of the leaves with respect to the stem, and ascertain 

 what curvatures and torsions have taken place. 



130. Geotropic Curvatures in Organs in which Growth in length, 

 has Ceased. It has been a mattef of common observation that the 

 branches of many trees, two, three or more years old, undergo 

 upward curvatures. Such curvatures have been observed in 

 branches of many conifers 4.5 cm. in thickness and progeotropic 

 curvatures in the weeping branches of many trees have also been 

 seen. Jost records the apogeotropic curvature of a branch of a 

 fir seven years old 1 (Fig. 38). 



•Vochting, H. Organbildung im Pflanzenreich. 2 185. 1884. 

 Jost, L. Ueber einige Eigenthiimlichkeiten des Cambiums der Baume. Bot.. 

 Zeitung, 59 : I. 1901. 



