230 



MOVEMENT IN PLANTS. 



(2) What happens if a shoot, laid horizontally, is fixed 

 so that it cannot curve upwards ? Lay a few Bean seed- 

 lings on moist sawdust and keep the shoots down with a 

 piece of thick glass, or fix them to a sheet of cork by 

 means of crossed pins, and set them in moist air for about 

 six hours. Then remove the glass or the pins : the shoot 

 will quickly bend upwards. How can you explain this 

 result ? 



302. Diageotropism and Exotropism of Rootlets. 



As seen in 292, lateral roots of the first order take up 

 a definite angle a few degrees below the horizontal 

 position to which they return when the plant is tilted. 

 The rootlets of the second and higher orders grow in 

 various directions; instead of being geotropic they are 

 exotropic, tending to grow away from their parent 

 root. 



(a) Instead of tilting up the glass-sided box through 

 45, turn it upside down, after tying cloth or wire-gauze 

 over the soil (or in any other way preventing it from 

 falling out) ; note that the secondary roots grow downwards 

 until they have reached their original angle below the 

 horizon. After two or three days, turn the box round into 

 its original position, and note the further curvature made 

 by these roots in order to resume their original direction 

 of growth, thus becoming S- shaped. 



(b) Grow a Bean seedling in the glass-sided box until 

 it has a well-developed root system with diageotropic 

 secondary roots and exotropic tertiary roots. Then cut 

 off the terminal portion of the primary root, at from 

 2 to 4 cm. behind the tip. Note that (a) one of the 

 young secondary roots curves downwards and behaves 

 like the primary root forming a new "tap-root" or 

 " leader " ; (6) the branches borne by this " promoted " 

 root are diageotropic instead of exotropic, and these in 

 turn bear exotropic rootlets. 



303. Diageotropism in Stem Branches. Take four 

 potted plants of any kind, which have several lateral 

 branches. In each case note the positions of the branches 



