DIV. ii PHYSIOLOGY 343 



attained, the plant tends towards it by upwardly or downwardly directed movements, 

 and then takes on the horizontal growth. The oblique position naturally assumed 

 by many organs is in part the result of other influences. 



A special form of geotropic orientation is manifested by dorsiventral organs, e.g. 

 foliage leaves, zygomorphic flowers (p. 72). 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. In the 

 orientation of dorsiventral organs, not merely simple curvatures but torsions are 

 concerned. 



The rotation of the ovaries of many Orchidaceae, of the flowers of the 

 Lobeliaceae, of the leaf-stalks on all hanging or oblique branches, of the reversed 

 leaves (with the palisade parenchyma on the under side) of the Alstroemeriae, 

 and of Allium ursinum, all afford familiar examples of torsion regularly occur- 

 ring in the process of orientation. 



The foliage leaves which possess pulvini must again be specially mentioned 

 among dorsiventral organs since they can change their position by geotropic 

 variation movements in the fully-grown state. 



Twining" Plants ( 102 ), which are found in the most various families 

 of plants, have shoots which require to grow erect but are unable to 

 support their own weight. The erect stems of other plants, which 

 often secure their own rigidity only by great expenditure of assimi- 

 lated material (in xylem and sclerenchyma), are made use of by 

 stem-climbers as supports on which to spread out their assimilatory 

 organs in the free air and light. The utilisation of a support pro- 

 duced by the assimilatory activity of other plants is a peculiarity they 

 possess in common with other climbers, such as tendril- and root- 

 climbers. Unlike them, however, the stem-climbers accomplish their 

 purpose, not by the help of lateral clinging organs, but by the 

 capacity of their main stems to twine about a support. The first 

 internodes of young stem-climbers, as developed from the subterranean 

 organs which contain the reserve food material, as a rule stand erect. 

 With further growth the free end curves energetically to one side and 

 assumes a more or less oblique or horizontal position. At the same time 

 the inclined apex begins to revolve in circles like the hand of a watch. 

 This movement continues from the time of its inception as long as the 

 growth of the shoot lasts, and as a rule takes place in a definite direc- 

 tion. In the majority of twining plants the circling movement as seen 

 from above is in the direction opposite to that of the hands of a watch 

 (towards the left as we commonly express it). The Hop and the 

 Honeysuckle twine to the right, in the direction of the hands of a 

 watch. In Bowiea volubilis and Loasa lateritia a rotation alternately to 

 the right and left has been observed. The plants that circle to the 

 left are also left-handed climbers, i.e. the spiral which their stems 

 form (Fig. 281 /) mounts from the left to the right and, as seen 

 from above, against the direction of the hands of a watch. Similarly 

 the plants that circle to the right are right-handed climbers. There is 

 thus a close relation between the revolving movement and the twining. 



