250 TROPIC MOVEMENTS 



ascending position is in many cases due to diageotropism, which may either 

 persist unaltered in light and darkness, as in Fragaria vesca and F. grandi- 

 flora, or may change more or less completely into negative geotropism 

 in darkness, as in Lysimachia nummularia, Polygonum aviculare, Rubus 

 caesius, Vinca major, and S tacky s sylvatica, so that the shoots are nearly erect 

 in darkness or when growing in thick grass and obliquely ascending when 

 illuminated. Hence in sunny situations the shoots of these plants are 

 pressed against the ground even when this necessitates a downward 

 curvature. 



The power of changing the geotropic tone varies according to the 

 degree of development and morphological rank of the organs, although a day 

 or two is sufficient under favourable conditions to produce a reversal in 

 the active growing zone. Thus in Glechoma hederacea the runners formed 

 in spring show a pronounced geotropic erection in darkness, whereas those 

 formed later in the season show none at all *. It was owing to this fact 

 that Czapek could detect no change of position in the runners of this plant 

 and also of Potentilla reptans in darkness, whereas Maige 2 found that the 

 last-named plant also became negatively geotropic in darkness. 



In certain plants the geotropic tone may be modified by changes 

 of temperature, and it is for this reason that the ascending shoots of 

 Veronica chamaedrys and Lamium purpureum sink to a more or less 

 horizontal position when the temperature is kept low. The photic and 

 thermal changes of geotropic tone will be opposed when doubly responsive 

 plants are subjected to simultaneous rises of temperature and of illumination. 



Frank was the first to recognize that permanently diatropic positions were 

 due to diageotropism, but supposed that the changes of position according 

 to the illumination were due to negative geotropism and variable negative 

 heliotropism. Czapek 3 , however, showed that the same changes of position 

 took place in homogeneous diffuse light, but not when the plants were 

 rotated on a klinostat. When the action of gravity is eliminated in this 

 way the shoots of Lysimachia nummularia and other plants are able to 

 show feeble positive heliotropism 4 , whereas the creeping shoots of a few 

 other plants are feebly negatively heliotropic 5 . 



The runners and creeping shoots of most of these plants are originally 

 physiologically radial, and only acquire a temporary dorsiventrality after 



Sitzungsb. d. Wien. Akad., 1895, Bd. civ, Abth. i, pp. 1234, I2 49J Oltmanns, Flora, 1897, p. 24; 

 Maige, Ann. d. sci. nat., 1900, 7 ser., T. xi, p. 334 ; Massart, L'initabilite" d. plantes superieures, 



1902, p. 13. 



1 Maige, 1. c. ; Oltmanns, 1. c., p. 25 ; Klebs, \Villkiirliche Entwickelungsanderungen bei Pflanzen, 



1903. On the influence of external and internal conditions on the formation of runners cf. Maige, 

 1. c. ; Goebel, Organography, 1905, p. 459. 



a Maige, I.e., p. 340. 



3 Czapek, 1. c., p. 1235. Cf. also Oltmanns and Maige, 1. c. * Czapek, 1. c., p. 1236. 



8 Maige, 1. c., p. 358. 



