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1920] SCHLEY— GEO-PRESENTATION AND GEQ-RE ACTION 71 



Copjeland (3) was unable to detect a difference in the opposite 

 flanks of stems split lengthwise and stimulated geotropically four 

 days. Kerstan (12), using the plasmolysis method, found no 

 increase of turgor in either flank of geotropically or heliotropically 

 stimulated shoots either during curvature or after its completion. 

 He concludes with Noll that the decrease of tu 

 fact that the osmotic producing substances do not keep pace with 

 the intake of w r ater of the cells and their increased volume. 



Thate (20) found Kraus's method too crude to determine the 

 difference of water in the two flanks of heliotropically stimulated 

 shoots, although he does not dispute its existence. On the other 

 hand, Tondera (21) was able to verify Kraus on this point and 

 from his studies developed the law: "As the cells of the rind 

 parenchyma of the lower organ half become filled by the streaming 

 of water, due to gravity, the cells of the opposite half become 

 water-poor, the resulting difference in pressure forcing the organ 

 to move toward the water-poor half. " This at best is a very crude 

 conception. 



From the cytological standpoint, McDougal (15) found that 

 the cells of the convex side are greater in length, breadth, and 

 thickness than those of the corresponding tissue of the concave 

 side of geotropically stimulated roots. Georgevitch (8) confirms 

 this earlier work, while Buchner (i) found the same condition 

 in shoots that had been prevented from responding to gravity 



stimulation. 



Czapek (4) is probably the chief worker in the chemical tield 

 of geo-presentation and reaction. Working with normal seedlings, 

 he found that homogentisic acid is produced as a product of the 

 oxidation of tyrosin, through the action of an oxidase, tyrosinase. 

 In geotropic stimulation the tyrosin is converted into homogentisic 

 acid by tyrosinase, as in normal seedlings, but the further oxidation 

 of the homogentisic acid by the oxidase is inhibited by the produc- 

 tion of an anti-oxidase, which renders the oxidase partly ineffective 

 and by this means causes an accumulation of homogentisic acid. 

 The accumulation begins after five minutes' stimulation, reaches 

 a maximum at the time of distinct curvature, and disappears when 

 reaction is complete. Grottian (10) and Grafe and Linsbauer 

 (9), however, were unable to confirm his results. They found as 





i 



