THE PROGRESS AND MODE OF MOVEMENT 231 



in unicellular organs. Any awakening of growth will naturally restore 

 the power of nutation curvature, and it is for this reason that grass-stems, 

 which have ceased to grow while erect, perform an upward geotropic 

 curvature when placed horizontally 1 . That two reactions are involved 

 is shown by the fact that on the klinostat, when the action of gravity is 

 uniformly distributed, no geotropic curvature is produced, whereas the 

 awakening of growth 2 enables a heliotropic response to be made to 

 unilateral illumination. 



According to Barth 3 , the stem-nodes of Dianthus bannaticus behave 

 similarly, while Miehe 4 found that the adult nodes of Tradescantia 

 fluminensis remained capable of geotropic response. In most cases, however, 

 the heliotropic and geotropic irritabilities appear to be lost with the normal 

 cessation of growth. Before this happens tropic stimulation may often 

 cause a more or less marked acceleration of growth, such as is also shown 

 when those parts of tendrils where growth has fallen to a minimum are 

 subjected to contact stimu- 

 lation. Leaves, especially 

 when they possess pulvini, 

 may, however, remain capable 

 of heliotropic and geotropic 

 response for weeks or months 

 after the leaf appears to be 

 fully grown 5 . Preuss even 

 found that a leaf of Codiae- 

 um Wendlandi eight months 



Old remained Capable Of re- FIG. 44. Portion of haulm of TrUicutn vulgare showing the 



_ T , . , . or eotropic curvature produced twenty-four hours after it had been 



action. Ultimately the power placed in a horizontal position. 



of reaction is lost in all 



cases, and even in the nodes of grasses the power of renewed growth is 

 not indefinitely retained. The total amount of growth is in all cases 

 limited, and hence a grass-node can only perform one or two geotropic 

 curvatures. The production of two successive curvatures in opposed 

 directions appears, however, to result in a greater total growth than when 

 the unilateral action of gravity is eliminated on the klinostat. 



Usually the power of geotropic curvature is restricted to the normal 



1 Sachs, Arb. d. hot. Inst. in Wiirzburg, 1872, Bd. I, p. 204; de Vries, Landw. Jahrb., 1880, 

 Bd. ix, p. 473; Pfeffer, Druck- und Arbeitsleistungen, 1893, p. 390; Barth, Die geotropische 

 Wachsthumskriimmung der Knoten, 1894, p. 30. 



2 Elfving, Ueber das Verhalten d. Grasknoten am Klinostat, 1884 (Ofvertryck af Finska 

 Vetenskaps Societetens Forhandlingar, Bd. xxvi) ; Barth, 1. c., p. 33. 



3 Barth, 1. c., p. 27. 4 Miehe, Jahrb. f. wiss. Bot., 1902, Bd. xxxvil, p. 532. 

 6 Mobius, Festschrift f. Schwendener, 1899, p. 40; Preuss, Die Beziehungen zwischen dem 



anat. Bau und d. physiol. Function d. Blattstiele und Gelenkpolster, 1885. Cf. also Frank, Die 

 natiirl. wagerechte Richtung von Pflanzentheilen, 1870, p. 50. 



