CHAPTER XVIII. 



MOVEMENT IN PLANTS DUE TO IRRITABILITY. 



183. Movement in response to stimulus, Beside the growth 

 movements which take place in plant parts, the parts of plants 

 show certain movements which are due to irritability. In this 

 kind of movement the plant is influenced by some exciting cause, 

 called a stimulus. The stimulus acts upon the irritable part of 

 the plant, and in response to this movement occurs. We can 

 easily study the effect of several different kinds of stimuli. 



184. Influence of the earth on the direction of growth. In 

 the germination of the seeds which we have used in some of the 

 earlier experiments it has probably been observed that the direc- 

 tion which the root and stem take upon germination is not due 

 to the position in which the seed happens to lie. Under normal 

 conditions we have seen that the root grows downward and the 

 stem upward. 



Exercise 39. 



185. To study the influence of the earth on roots. Take seedlings grown 

 in a germinator which are free from the soil. Pin several seedlings to a cork 

 in such a way that the stems and roots of different ones will be lying in 

 different directions. Mark off the tip of the root of several with ink, as in 

 paragraph 176. Cut off the extreme tip from a few of the roots. Place the 

 cork in a moist chamber, with an abundance of water or saturated paper in 

 the bottom. On the following day observe the positions of the roots and 

 stems. Sketch and annotate. In the case of the roots marked into millimeter 

 spaces determine the motor zone (region of curvature) of the root. Comparing 

 these with the roots from which the tip was cut determine the perceptive zone 

 (the zone which receives the stimulus). Now turn the cork in another posi- 

 tion, leave for a day and note the result. 



107 



