MOVEMENT IN PLANTS. 227 



lines 2 mm. apart, starting from the tip. Place the 

 seedlings horizontally in a moist chamber or glass- sided 

 box, and after a day or two measure the distances between 

 the marks, the root having curved downwards. The 

 region of greatest curvature corresponds to that of greatest 

 growth in length ; the curvature first becomes evident in 

 the second zone from the tip, appearing later in the zones 

 farther back, and the most active curvature is usually in 

 the third zone. 



295. Oxygen necessary for Geotropic Curvature. 



(a) Take several Bean seedlings with roots 3 or 4 cm. 

 long, and pin them, with the radicles vertical, to the lower 

 sides of the corks of two jars (A, B), using long pins. 

 Fill A with water, and push the cork in tightly ; put a 

 little water in B, to keep the air moist, and cork loosely 

 or leave open ; then place each jar on its side, so that the 

 roots lie horizontally, and prop them in this position. 

 The roots in A make no curvature, or very little, owing to 

 lack of oxygen ; those in B make the usual downward 

 curvature. 



(b) Pin in a number of seedlings horizontally to a strip 

 of wood, set the strip vertical in a bottle half filled with 

 water that has been boiled and cooled, so that half of the 

 seedlings are in the water and the rest above it, and put 

 wet cotton- wool above the mouth of the jar on either side 

 of the strip of wood. In the lower (submerged) seedlings, 

 the roots continue to grow horizontally, but in those above 

 the water and therefore in damp air the roots soon curve 

 downwards. 



296. Effect of Removal of Boot-tip. (a) Take twelve Bean 

 seedlings with roots 2 to 3 cm. long, and mark each root with a 

 transverse Indian-ink line at 1 "5 cm. from the tip. Amputate the 

 tips of the roots in all except three of the seedlings by making a 

 transverse cut with a razor ; from three of the roots remove 1 mm. , 

 from other three roots remove 2 mm., and from the remaining three 

 roots remove 3 mm. Lay the twelve seedlings in moist sawdust 

 or sphagnum, with the roots horizontal. Note that the three intact 

 roots grow more actively than the nine amputated roots, and make 

 the normal downward curvature. If the decapitated roots make 

 curvatures, these will be in various directions sideways or even 



