90 PLANT PHYSIOLOGY 
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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 
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