SECT. II PHYSIOLOGY 311 



direction and afterwards maintain it. Fornierl}-, it was believed that this resulted 

 solely from their weight and the pliancy of their tissues. It is now known that 

 this is not the case, and that positively geotropic, like negatively geotropic, 

 movements are possible only through growth. The power of a downward curving 

 root-tip to penetrate mercury (specifically much the heavier), and to overcome the 

 resistant pressure, much greater than its own weight, proves conclusively that 

 positive heliotropism is a manifestation of a vital process. Positive geotropic 

 curvature is due to the fact that the growth, of an organ in length is 



PROMOTED ON THE UPPER SIDE, AND RETARDED, EVEN MORE STRONGLY, ON THE 



SIDE TURNED TOWARDS THE EARTH. Fig. 2-32 represents the course of the 

 geotropic curvature in a root. 



Most lateral branches and roots of the first order are plagiogeotropic, while 

 branches and roots of a higher order stand out from their parent organ in all direc- 

 tions. These 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 inclination they return to it by curving. 

 A special instance of plagiogeotropism is exhibited by strictly horizontal organs, such 

 as rhizomes and stolons, which, once they have attained their proper depth, show a 

 strictly tkansverse geotropism (diageotropism). Should the proper depth not be 

 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. 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 origin- 

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

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

 regularly occurring in the process of orientation (""'). 



The foliage leaves which possess pulvini must be specially mentioned again 

 among dorsiventral organs since they can change their position by geotropic 

 variation movements in the fully-grown state. 



As in the case of phototropism, an alteration of the relation of the 

 organ of the plant to the geotropic stimulus may be brought about by 

 internal or external influences. It is very common to find a change 

 of the geotropic position during the development of the shoots of 

 plants with rhizomes and also of flowers and fruits. In this way the 

 fruits of Trifolium subterraneum and of Arachis hypogaea become buried 

 in the soil. Of external influences may be mentioned the cutting off 

 of the supply of air, inducing a want of oxygen which causes some 

 roots and rhizomes to become negatively geotropic. Temperature also 

 modifies the position of the main and lateral axes of some plants. 

 Thus the stems of many of our spring plants lie prostrate Avhile the 

 temperature is low, but become erect as a result of their geo- 

 tropism at a higher temperature. The influence of light modifies 

 especially the geotropism of rhizomes and of lateral roots. 



