126 BOTANY FOR BEGINNERS CHAP. 



flowers in a flask and holding them in position by a plug of 

 cotton wool pushed into the neck of the flask. Fit the flask in 

 an inverted position on to a cork through which one end of a 

 tube (open at both ends) passes. The other end of the tube 

 dips into mercury on the top of which a strong solution of 

 caustic potash floats. As the flowers in the flask use up the 

 oxygen and give out carbon dioxide, since the latter is con- 

 tinually absorbed by the caustic potash solution, the mercury 

 rises in the tube. This is because of the diminution of the 

 pressure inside the flask. If the gas in the flask be tested, a 

 loss of oxygen becomes evident, and by weighing, the caustic 

 potash solution can be proved to have increased in weight. 

 If the temperature of the flask be noted a rise of temperature 

 will be shown. Fig. 146 shows the apparatus which can be 

 used for this experiment. 



Conditions necessary for Respiration. The conditions 

 under which respiration occurs may be stated as follows : 



1. Plants respire in both light and darkness. 



2. An atmosphere containing free oxygen is necessary. 



3. A certain temperature is most favourable. If seeds are 

 kept in a cold place they respire slowly, but if moved to a warm 

 place the respiration increases. 



Parts of Plants which Respire vigorously. The 

 parts of plants which stand in need of oxygen are : 



1. Every living cell at all times ; dead parts of plants 

 cannot respire. 



2. Germinating seeds which respire with great vigour. 



3. All parts which are growing actively. There is always a 

 rise of temperature due to the very active respiration at these 

 times. 



4. Developing flowers ; during the time of flowering there 

 is a great demand for oxygen, and a rise of temperature always 

 takes place. 



Use of Oxygen. Plants require oxygen for two purposes. 

 In the first place it is necessary for the building up of cellulose, 

 starch, sugar, proteids, and protoplasm. It is also necessary 

 for respiration, for without it the plant cannot gain the 

 necessary energy for the vital processes to be carried out. 



EXPT. 116. Steep a few Peas in water for twenty-four hours, and 

 place them on damp cotton-wool at the bottom of a bottle. Close the 



