xii PHYSIOLOGY OF GROWTH AND MOVEMENT 161 



to night. It is a general rule that flowers are open during light 

 and closed at night ; but a few open at night and close during 

 the day. The closing of the flowers during darkness is called 

 the sleep of the flowers. 



EXPT. 154. Collect a few flowers of the Dandelion, and place them 

 in a tin box so that they receive no light. Note 



(i) At the end of an hour they are all closed up. 



Now place their cut ends in water and expose them to a bright light, 



(ii) They will open again. 



This shows that the amount of light which they receive causes them 

 to open and close. 



EXPT. 155. Bring a Tulip plant with fully developed flowers, which 

 are closed, into a warm room. Note 



(i) If the temperature of the room is about ten degrees Fahrenheit 

 higher than the external air whence the plant was obtained, the flowers 

 open. 



Now expose the flowers to a lower temperature, either by placing 

 them outside in the cold, or by surrounding the pot with a mixture of 

 salt and ice. 



(ii) The flowers will close. The closing of the flowers is due to the 

 decrease in the amount of heat which they receive. 



The Sleep of Leaves. If the compound leaves of the 

 Wood Sorrel or Clover are examined during early morning they 

 are found to be folded so as to expose the least amount of sur- 

 face to the atmosphere. If the same leaves are noticed at noon 

 they will be seen to be fully expanded. 



The Utility of Plant Movements. The heliotropic 

 and geotropic movements of organs place them in the most 

 favourable position for performing their functions. 



Thus, when leaves are placed at right angles to the rays of 

 light, they receive the maximum amount of light and energy, 

 and are thus able to assimilate to perfection. The primary 

 root being positively geotropic, carries the secondary roots into 

 new soil from which food is obtained. The primary stem grow- 

 ing erect, places the aerial organs in a good position for receiving 

 light. The roots being both geotropic and hydrotropic, they 

 are placed in a good position to obtain food, and to fix the plant 

 firmly in the soil. The flowers close at night to protect the 

 internal organs from losing heat by radiation, and to prevent 

 them from being washed by rain and dew. They open in warm 

 sunshine so that insects can visit them, and close in the cold to 

 prevent loss of heat. Those flowers which open during dark- 

 ness and close during light are visited by night-flying insects. 



M 



