258 BOTANY 



curvature, as in the case of negatively geotropic stems, takes 



place. 



Diageotropism. — Most lateral branches and roots of the first order 

 are diageotropic, while branches and roots of a 

 higher order stand out from their parent organ 

 .in all directions. Diageotropic organs are 



ONLY IN A POSITION OF EQUILIBRIUM WHEN 

 THEIR LONGITUDINAL AXES FORM A DEFINITE 

 ANGLE WITH THE LINE OF THE ACTION OF 



GRAVITY. If forced from their normal inclina- 

 tion they return to it by curving. A special 

 instance of diageotropism is exhibited by 

 strictly horizontal organs, such as rhizomes and 

 stolons, which show a strictly transverse geo- 

 tropism, and, if removed from their normal 

 position, their growing tips always return to the 

 horizontal. A more complex form of geotropic 

 orientation is manifested by dorsiventral organs. 

 These, in contrast to radial organs, such as most 

 fig. 203. — Geotropic eurva- roots and stems, are not developed on a uni- 

 T^TZTTTI form plan on all sides, but show two usually 

 horizontally ; ii, after externally perceptible different sides — a dorsal 

 seven hours; in, after an( j a ventral side. The foliage leaves of 



twenty-three hours \ Z, a ,t,-tt t -i-n 



fixed index. (After Sachs.) m °st dicotyledons and zygomorphic flowers 

 {Antirrhinum, Accmitum, etc.) furnish pronounced 

 examples of dorsiventral structure. All such dorsiventral organs, 

 just as radial organs that are diageotropic, form a definite angle 

 with the direction of gravity, but are only in equilibrium when 

 the dorsal side is uppermost. If, in spite of the proper inclination of 

 the longitudinal axis, the dorsal side should lie underneath, it elongates 

 until it comes back again into a dorsal position. 



A state of torsion often results from the orientation movements of dorsiventral 

 organs to recover from abnormal positions. Similarly, a torsion must also, of neces- 

 sity, occur when a geotropic organ, which has become curved over toward its parent 

 axis, turns itself about so as to face outwards (exotropism). The rotation of the 

 ovaries of many Orchidaceae, of the flowers of the Lobeliaceac, of the leaf-stalks on 

 all hanging or oblique branches, of the originally reversed leaves (with the palisade 

 parenchyma on the under side) of the Alstroeineriae, and of Allium ursinum, all 

 afford familiar examples of torsion regularly occurring in the process of orientation. 



Stem - Climbers. — In addition to the better - known forms of 

 geotropism already mentioned, stem-climbers exhibit a peculiar and 

 only recently recognised geotropic movement, by means of which they 

 are enabled to twine about upright supports. This movement depends 

 upon the geotropic promotion of the growth of one side (not, as in 

 negative or positive geotropism, of the upper or lower portions). Thus 

 a geotropic curvature in a horizontal plane is produced (lateral geo- 



