SPECIAL AND GENERAL ACTIONS 143 



Oxygen. All aerobic organisms rapidly lose the power of movement 

 and curvature in the absence of oxygen 1 , but the rigor is not immediately 

 produced in the tentacles of Drosera, for Correns found that they remain 

 for a time responsive to mechanical and chemical stimuli, just as a muscle 

 does in the temporary absence of oxygen. Similarly the leaves of Mimosa 

 pudica may show a feeble power of seismonic movement immediately after 

 the oxygen pressure has been reduced almost to nil 2 . In any case, the 

 seismonic irritability of Mimosa pudica is lost at a lower partial pressure 

 of oxygen than the photonastic irritability which, in Mimosa as well as in 

 other plants examined by Correns, disappears in air at a pressure of 15 to 

 35 millimetres of mercury 3 . Tendrils cease to react to contact in air 

 at a pressure of 15 to 30 mm. of mercury, but their growth appears 

 to be still possible, for an induced movement continues to a slight extent 

 at still lower pressures in which the power of perception is lost. Correns 

 was, however, unable to observe any such after-effect when seedling-stems 

 were brought after geotropic or heliotropic induction into air sufficiently 

 rarefied to suppress the power of perception of these stimuli. The 

 seedling-stem of Helianthus animus, however, continues to grow for a time 

 in the absence of air 4 , and is able to perform a geotropic curvature in an 

 almost complete vacuum, whereas no heliotropic response is possible when 

 the air-pressure falls below 75 mm. of mercury. It has, however, not 

 been determined whether the power of aitionastic curvature in general 

 is lost sooner than the power of growth, or whether shock-stimuli become 

 ineffective at a higher pressure of oxygen than continuous stimuli. 



The seedling-stem of Sinapis alba is capable of a geotropic curvature 

 at an air-pressure of 30 to 37-5 mm. of mercury, but is unable to per- 

 form a heliotropic reaction below an air-pressure of 45 mm. Hence 

 below this air-pressure the stem is capable of a geotropic but not 

 of a heliotropic response. After exposure to geotropic or heliotropic 

 induction in air too rarefied to permit of any response, no after-effect is 

 shown on the return to ordinary air. Heliotropic induction, and to a less 

 extent geotropic induction, are therefore suppressed by a fall of the air-pres- 

 sure to limits which permit of growth and geotropic curvature, whereas no 

 heliotropic curvature follows previous stimulatory induction in ordinary 

 air. It follows, therefore, that in rarefied air not only is the power of 

 perception of heliotropic stimuli lost, but also the process of induction is 

 suppressed. 



The action of geotropic and heliotropic stimuli is little or not at all 



1 Correns, Flora, 1892, p. 87; Sachs, Flora, 1863, p. 501; Kabscb, Bot. Ztg., 1862, p. 341 ; 

 Dutrochet, Memoires d. ve"getaux et d. animaux, Bruxelles, 1837, pp. 186, 259. 



2 Correns, I.e., pp. 96, 144. 



3 Correns, 1. c., p. 117. 



4 Cf. Nabokich, Beiheft z. Bot. Centralbl., 1902, Bd. xm, p. 272. 



