90 PLANT PHYSIOLOGY 



available in every living cell and not confined to any 

 definite locality in the plant. This is made available by 

 the process known as respiration. 



135. Respiration. With the exception of a few 

 bacteria and low fungi to be mentioned later all living 

 cells absorb oxygen and give off carbon dioxide, the 

 process being accompanied by a loss in weight. In 

 green plants in the light the absorption of carbon dioxide 

 and giving out of oxygen are so much greater than this 

 other process that for years it was not known that the 

 latter takes place. It is not dependent upon the 

 presence of light nor are chloroplasts necessary for its 

 occurrence. It takes place more rapidly the higher the 

 temperature until an optimum temperature is reached 

 which is sometimes perilously near to the death point of 

 the cell. 



136. The oxygen is taken from the air (which contains 

 nearly 20 per cent, of oxygen) by the aerial parts of the 

 plant. It passes through the stomata and lenticels and 

 also to some extent through the cuticle into the inter- 

 cellular spaces and from thence is absorbed by the 

 cells. The roots whose outer walls are only slightly 

 cutinized and whose root hairs are practically free from 

 cutin absorb the oxygen which is dissolved in the soil 

 water and which is present in the air spaces between 

 the soil particles. Submerged plants, e.g. algae, absorb 

 the oxygen dissolved in the water. Many trees which 

 grow in swamps where the soil lacks oxygen send up 

 peculiar vertical branches from their roots out to the 

 surface and up into the air, these serving as aerating 

 organs for the roots. Such are the "knees" of the 

 bald cypress (Taxodium distichum) when the latter 

 grows in wet places (and which are lacking when it grows 

 in well aerated soil) and the aerial roots of some of the 



