CH. VIl] RESPIRATION. 1$3 



cences. We arrange the experiment differently. Cut 

 a groove in a block of cork about 5 mm. wide and 

 5 deep. Arrange 3 or 4 grass stalks on the cork so that 

 they lie across the groove with the pulvinus of each over 

 the groove. Fix them in place by two sheets of cork, one 

 of which pins down the stalks on one side of the groove, 

 while the second sheet fixes the parts of the stalks on the 

 opposite side. The whole arrangement is put in a tin 

 box half full of wet sawdust for 3 or 4 days, when the 

 result should be clear. 



(196) Free oxygen necessary. 



* 

 The bean which should have a short root (15 mm.) is 



pinned in the vertical position to the lower surface of a 



rubber cork fitting tightly into the ground neck of a 



small bottle. Hydrogen is passed through the bottle for 



about 2 hours to replace the contained air. The bottle is 



then placed on its side so that the root is horizontal. 



The tube connecting with the hydrogen-generator is kept 



open while the outflow tube by which the hydrogen 



current escaped from the bottle is shut; in this the 



only leakage that can occur is an escape of hydrogen. 



After 24 hours, during which no geotropic curvature 



should occur, the hydrogen is replaced by air, when the 



root will bend downwards. 



(197) Free oxygen, necessary. 



The same result may be more simply obtained by 

 keeping seedling beans completely submerged while 

 control specimens are in damp air, or just touching the 



