MOVEMENT IN PLANTS DUE TO IRRITABILITY. 1 09 



of the shoots will soon turn upward again. Young bean plants 

 growing in a pot began within two 

 hours to turn the ends of the shoots 

 upward. 



Horizontal leaves and shoots can 

 be shown to be subject to the same in- 

 fluence, and are therefore diageoiropic. 

 189. Influence 

 of light. Not 



Fig. 85. 



Pumpkin seedling showing apogeotropism. Seedling at the left placed hori- 

 zontally. In twenty-four hours the stem has become erect. 



only is light a very important factor for plants during starch 

 formation, it exerts great influence on plant growth and 

 movement. 



Demonstration 29. 



190. To prepare plants grown in the dark Three or four weeks be- 

 fore these plants are wanted for study the teacher may plant a sufficient 

 number of seeds (radish or other seeds) in small pots for the class to study. 

 Several different kinds of seeds may be used for comparison if desired. 

 Place one lot of the pots in a warm but very dark place. They may be put 

 in a box, and the box can be then covered with two or three layers of black 

 cloth, sufficient to shut out all light. Keep the box in a warm room, and oc- 

 casionally open it to water the plants if necessary. The lot kept in the 

 light should have the same temperature conditions. If preferred the pupils 

 can plant the seeds, and place those to be grown in the dark in a common 

 box. This is preferable if it is convenient for the pupils to do it. 



Exercise 41 . 



191. Influence of light on the growth of plants. When the plants have 

 grown for about two weeks they will be ready for study. Compare the 

 plants grown in the dark with those grown in the light. Which lot have 

 the longer stems ? What influence then does light have on growth in 



