94 AERATION AND AIR-CONTENT. 



tentacles of Drosera, but the power to perceive stimuli and to re- 

 spond persisted in the presence of the minutest quantities of oxygen. 



Czapek (1895 : 274) showed that geotropic induction is possible 

 in lupine seedlings placed in a vacuum and then in hydrogen, al- 

 though no growth took place. 



Paal (1912 : 1) noted the time required for the geotropic curvature 

 of bean roots at different air-pressures, and found that the reaction- 

 time was lengthened by reduced pressure. Further experiment 

 showed that the presentation-time was also increased by decreasing 

 pressure, gradually at first and then very rapidly. 



Kenkel (1913) studied the effect of reduced air-pressure upon irri- 

 tability, and found that the geotropic reaction still occurred at an 

 oxygen-content at which heliotropic curvature was no longer possible. 



Van Ameijden (1917 :211) has carried out a comprehensive inves- 

 tigation as to the irritability of Avena saliva in an oxygen-free 

 atmosphere obtained by means of nitrogen. He finds that when 

 seedlings, long enough removed from the effect of oxygen, receive geo- 

 tropic or heliotropic stimuli in the absence of oxygen and are placed 

 at once in the air, they are unable to react. Reaction does occur, 

 however, after a similar fore-period, if the seedlings are both stimu- 

 lated and left in the air. In the absence of oxygen the perception of a 

 stimulus can not take place if there has been a sufficiently long fore- 

 period. After the perception of a stimulus the seedlings do not 

 react if left in an oxygen-free atmosphere, showing that oxygen is 

 necessary for the reaction. Seedlings retain the power of perception 

 for a long time in a low oxygen-content, but this is weakened by a 

 prolonged stay. Complete or partial withdrawal of oxygen produced 

 no difference in the reaction of seedlings to geotropic or heliotropic 

 stimuli. 



Summary. Practically all tropistic responses are suppressed in the 

 absence of oxygen, though reaction to contact may persist in the 

 presence of minute quantities. The relation of geotropism to oxy- 

 gen has been studied by Wortmann, Detmer, Kraus, Correns, 

 Czapek, Ritter, Paal, Kenkel, and Van Ameijden, and all are in 

 agreement that geotropic curvature is impossible without oxygen, 

 whether this is secured by means of a vacuum or by the use of hydro- 

 gen, carbon dioxid, or nitrous oxid. Wiesner, Detmer, Correns, 

 Kenkel, and Van Ameijden have shown that heliotropic response is 

 similarly inhibited. The nyctotropic movements of leaves and the 

 anthotropic ones of flower-heads have been found by Dutrochet and 

 Kabsch to cease in a vacuum, as well as under greatly reduced pres- 

 sure or in carbon dioxid. Correns obtained the same result with the 

 leaves of several genera under reduced pressure, or in pure hydrogen, 

 nitrogen, carbon dioxid, or nitrous oxid, and both Kabsch and Cor- 

 rens observed that the irritability of stamen filaments was suppressed 

 by carbon dioxid. 



