PHYSIOLOGY 



281 



i-asr of a root of the Broad Bean, which was marked at regular intervals with Indian 

 ink and placed horizontally. 



Diageotropism. Most lateral branches and roots of the first order are diageo- 

 tropic, while branches and roots of a higher order stand out from their parent organ 

 in all directions. DIAGEOTKOPIC ORGANS ARE ONLY IN A POSITION OF F.^UI- 



I.IIIRIt'M WHEN THEIR LONGITUDINAL AXES FORM A 

 DEFINITE ANGLE WITH THE LINE OF THE ACTION OF 



GRAVITY. If forced from their normal inclination they 

 return to it by curving. A special instance of diageo- 

 tropism is exhibited by strictly horizontal organs, such 

 as rhizomes and stolons, which, once they have attained 

 their proper depth, show a strictly TRANSVERSE GEO- 

 TROPISM, and, if removed from their normal position, 

 their growing tips always return to the horizontal. 

 The oblique position naturally assumed by many organs 

 is in part the result of other influences (epinasty and 

 hyponasty, their own weight, and other directive forces). 

 A more complex form of geotropic orientation is 

 manifested by dorsiventral organs, e.y. foliage leaves, 

 xygomorphic flowers. All such dorsiventral organs, 

 just as radial organs that are diageotropic, form a definite 

 angle with the direction of gravity, but arc only in 

 equilibrium when -the dorsal side is uppermost. If, in 



spite of the proper inclination of the longitudinal axis, Flli ; 



ture of the root < it ;i seedling 



the dorsal side should he underneath, the latter elon- 

 gates until it comes back again into a dorsal position. 



W 



of Vma Fl<. I, Placed 

 liorixnntally : II- after 

 seven hours ; ///. after 

 twenty-three hours ; /. .1 

 fixed index. (After SA. !!*.) 



A state of torsion often results from the 

 orientation movements of dorsiventral organs 

 to recover from abnormal positions. Similarly, a torsion must also, 

 of necessity, occur when a geotropic organ, which has become curved 

 over toward its parent axis, turns itself about so as to face outwards 

 (EXOTROPISM) (Fig. 227). 



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 origin- 

 ally reversed leaves (with the palisade parenchyma on the under side) of the 

 Alstroemeriae, and of Allium ursinum, all atford familiar examples of torsion 

 regularly occurring in the process of orientation ( 85 ). 



Twining Plants. In addition to the better-known forms of 

 geotropism already mentioned, stem -climbers exhibit a peculiar 

 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 GEOTROPISM), 

 resulting in a revolving motion of the shoot apex. Twining plants 

 occur in very different plant families ; and although an upward 

 growth is essential to their full development, their stems are not able 



