122 



GROWTH AND WORK OF PLANTS 



replaced in the ordinary* respiration of plants by the carbon 

 which is obtained from the carbon dioxide absorbed in the mak- 

 ing of sugar and starch. 



196. Respiration causes waste of plant substance. In 

 paragraph 189, we learned that respiration is a breaking down 

 process since the carbon dioxide comes from the substance of the 

 plant or animal. It is one of the forms of waste which comes 

 from certain kinds of work performed by the plant or animal, 

 but energy is given off. In hard physical work, or in strenuous 

 games, as in running, rowing, or football, breathing is very rapid 

 and the waste often becomes very great in a short time, though 

 it is not all due to respiration. To repair this waste the plant or 

 animal must have food to build up the new living substance. 



197. Contrast of respiration and photosynthesis. In con- 

 trast with this breaking down process of respiration is the building 

 up process in plants from photosynthesis which we studied in 

 Chapter XIII. These two processes in the plant may be com- 

 pared in the following table: 



Carbon dioxide is taken in by the plant and 

 oxygen is liberated. 



Starch is formed as a result of the metabo- 

 lism or chemical change. 



The process takes place only in green plants, 

 Photosynthesis. and in the green parts of plants, that is, 

 in the presence of chlorophyll (excep- 

 tion in certain bacteria). 



The process only takes place under the influ- 

 ence of sunlight. 



It is a building up process, because new plant 

 substance is formed and energy is stored. 



* By ordinary respiration is meant aerobic respiration. Aerobic respi- 

 ration is carried on by plants which require air. Anaerobic respiration is 

 respiration in the absence of air. All plants can carry on anaerobic res- 

 piration for a short time, but most plants are injured and soon die or rest 

 if air is not accessible. Some plants are active either as anaerobes or aerobes 

 (the yeast plant), while others only as anaerobes (certain bacteria). 



