and Laboratory Methods. 



2465 



must be kept in the dark-room. Begin to make observations the third day after 

 planting. If properly managed, the roots of the seedlings will be deflected from 

 the vertical and follow the surface of the moist cone as they grow downward. 



Another method of demonstrating the same response to moisture employs a 

 moist cylinder, around which the seedlings are suspended. Cover a tall glass 

 cylinder, e. g., a Welsbach lamp chimney, with moist filter paper and stand 

 it erect in a battery -jar in which there is a shallow layer of water. Upon the top 

 of the cylinder lay a disk of thin wood or sheet cork having a row of holes around 



Fig. 20. — A centrifuge fitted for a demonstration of the rheoiropic behavior 



of roots. 



the circumference about 5 or 6 mm. from the outer surface of the cylinder. Insert 

 in the holes seedlings of white mustard or radish, arranging them parallel to the 

 surface of the cylinder. If the proper relations of moisture are maintained, the 

 results will be the same as indicated above. 



RHEOTROPISM. 



When exposed to the force of a current of water, the roots of many seedlings 

 give a very definite response by curvatures. ^ The most convenient way of con- 

 ducting the experiment is to allow the roots to dip in a revolving dish of water. 

 Figure 20 illustrates the method of revolving the dish on the horizontal plate of 



1 Cf. Newcombe. F. C. The Rheotropism of Roots. Bot. Gaz. 33: 177, 1902. 



