228 MOVEMENT IN PLANTS. 



upwards more often than downwards. Hence the removal of the 

 root-tip at any rate interferes with the normal downward geotropic 

 curvature. 



(b) The preceding experiment is not conclusive, but it suggests 

 further simple experiments. Take twelve Bean seedlings with roots 

 about 2 cm. long, and lay them horizontally in moist sawdust or 

 sphagnum. After an hour, leave two of the seedlings undisturbed 

 (A) ; turn two of them round so that the root points vertically 

 downwards (B) ; remove the others (C) and cut off their root- tips 

 (at 2 mm. from apex), then place them vertically. In A the root 

 curves downwards ; in both B and C the root curves sideways 

 (towards the side that was downwards while the root was laid 

 horizontally). These results show that (1) the root on being shifted 

 from the horizontal to the vertical position proceeds at first to carry 

 out the curvature induced in it while it was horizontal ; (2) re- 

 moval of the root-tip makes no difference to the carrying-out of this 

 induced curvature. 



297. Apogeotropism. The apogeotropic (negatively 

 geotropic) curvature of the shoot will have been noticed 

 in the preceding experiments, as contrasted with the 



ritively geotropic curvature of the radicle of seedlings, 

 may be demonstrated in various ways. (1) Lay a pot 

 of seedlings, or a potted plant, on its side ; or invert the 

 pot, after securing the soil from falling out. (2) Fix a 

 cut shoot in the split or bored cork of a bottle or test-tube 

 filled with water and laid horizontally. (3) Fix a shoot 

 into a sloping bank of wet sand in a box ; one end of the 

 box may be replaced by a glass sheet, so that the changes 

 in position of the shoot may be readily traced on the glass. 



298. Region of Geotropic Curvature in Stem. 



Mark the epicotyl of a Bean seedling, or the hypocotyl of 

 a Sunflower or Castor Oil seedling, at intervals of 10 mm., 

 starting from the tip. After twelve hours of horizontally, 

 note the form of the curved stem, and measure the 

 distances between the marks. The strongest curvature 

 takes place in the region of greatest growth. Later, 

 however^ when the stem has become erect, the greatest 

 curvature is at the base of the growing region, and it 

 continues until the upper part of the stem is carried 

 beyond the vertical, to which it returns at a still later 

 stage. 



