RESPIRATION AND OXYGEN. 31 



well-aerated water, peas grew downward, regardless of their original 

 position. When the aeration was stopped, growth ceased or went 

 on but slowly. If the stagnant oxygenless water was aerated, the 

 radicles grew downward into it, in spite of the carbon dioxid in it, 

 thus indicating that the previous curvature was a response to oxygen. 

 In consequence, the term oxytropic was proposed for this type of 

 irritability. Similar experiments with hemp and wheat gave the 

 same results, though hemp showed less marked response. 



Later studies (1896 : 191) with seedlings of hemp and peas in 

 tubes filled witht boiled sterilized water and closed with a plug of 

 cotton confirmed the existence of oxytropic curvature. The primary 

 roots of sunflower were strongly oxytropic, the secondary ones but 

 slightly so, while the radicles of Cucurbita showed distinct oxytro- 

 pism. Wieler (1898) has studied the pneumatodes of Phoenix and 

 Chamcerops and finds that in some respects they correspond to the 

 aerenchym of Schenck, but he was unable to find any proof that they 

 were definitely related to aeration. 



Pfeffer (1900 : 182) stated that oxytropic reactions are feeble and 

 that it is yet to be shown that they play a prominent part in the 

 orientation of roots in water and soil. He also regarded it as uncer- 

 tain whether the upward growth of roots in mud or water-logged 

 soil is due to oxytropism or to an alteration of geotropic irritability 

 produced by the deficiency of oxygen. The absence of oxygen pro- 

 duces disturbances of growth which often result in irregular curva- 

 tures. When curvature is toward the region with more oxygen, 

 growth is more rapid. It appears, however, as though the avoidance 

 of regions poor in oxygen is in part aided by the suppression or rever- 

 sal of the geotropic irritability, for on repeating Ewart's experiments 

 on a klinostat he was unable to obtain constant and definite curva- 

 tures away from the deoxygenated region. 



Bennett (1904 : 241) repeated Molisch's studies of the behavior 

 of roots to different gases, employing his methods, and came to the 

 conclusion that the curvatures noted were due to hydrotropism. In 

 further experiments, roots of Zea mays, Pisum sativum, Raphanus 

 sativus, Cucurbita pepo, and Lupinus albus were subjected to one- 

 sided access of oxygen, hydrogen, and carbon dioxid to determine 

 the presence of aerotropism. When the roots were grown in water 

 between submerged chambers containing air on one side and C0 2 or 

 H on the other, no constant or regular curvatures occurred, as was 

 also true when they were placed in a similar position in a damp 

 chamber. Similar results were obtained when the roots were grown 

 in a thin vertical layer of earth separating air and C0 2 or air and 

 hydrogen, or in earth with air on one side and carbon dioxid or hydro- 

 gen on the other, or in gelatin under similar disposition of the air 

 and gases. The conclusion is reached that, in so far as the repre- 

 sentative land plants used were concerned, definite curvatures are 



