MOVEMENT IN PLANTS DUE TO IRRITABILITY. 113 



about 35cm high were covered for nearly two days, so that the 

 light was excluded. At midday on the second day the box was 

 removed, and the leaves on the covered plants are well repre- 

 sented by fig. 90, which was made from one of them. The 

 leaves of the other plants in the box which were not covered 

 were horizontal, as shown by fig. 91. Now on leaving these 

 plants, which had exhibited induced "sleep" movements, 

 exposed to the light they gradually assumed the horizontal 

 position again. 



Synopsis. 



Irritability. 



Plants are irritable, that is, they respond to certain stimuli. 

 The force of gravity stimulates the tip of the root, and 

 causes the root to turn downward. 



The " motor zone," in response to this stimulus, is co- 

 incident with the region of elongation of the root. 

 The perceptive zone is in the root tip. 



The force of gravity stimulates the stem to turn upwards 

 (or away from the earth). 



f Progeotropism (in first root). 

 Geotropism. ? Diageotropism (in lateral roots)* 



( Apogeotropism^in stems). 

 Stems( horizontal stems are diahelio- 

 tropic) grow towards the light (heli- 

 otropic). 



Leaves turn so as to face the light (un- 

 less the light is very strong, when 

 they may turn their edge toward 

 the light). 



Light retards growth of stems, since 

 Influence of light. stems grown in the dark are longer. 

 Plants do not "sleep"; when the 

 leaves turn downward at night it is 

 because the influence of light is re- 

 moved and the leaf is free to turn in 

 the direction caused by growth,, the 

 growth being more active usually on 

 the upper side of the leaf after it 

 pushes out from the bud. 



