ANAEROBIC RESPIRATION 539 



plants continue to evolve carbon dioxide, thus indicating the occurrence of 

 respiration. Respiration of this tj^pe, proceeding in the absence of atmospheric 

 oxygen, is called anaerobic respiration. The principal conditions under which 

 anaerobic respiration may or does occur in tissues of higher plants will be dis- 

 cussed briefly. 



Many species of plants are adversely affected by flooding of the soils in 

 which they are rooted (Chap. XVII). Submergence of bottomland corn 

 fields — a frequent occurrence in some regions — soon results in serious damage 

 to the plants. This takes place even if only the roots are immersed. Main- 

 tenance of a saturated condition in the soil in which corn (maize) plants are 

 rooted for only a few days results in a yellowing and marked stunting of the 

 plants, and persistence of a flooded condition of the soil for a much longer 

 period soon results in their death. The plants often exhibit many of the 

 symptoms of desiccation, showing that the normal physiological processes of 

 the roots have been disturbed, and that absorption of water is no longer oc- 

 curring at an adequate rate. 



The roots of maize apparently require a relatively high partial pressure 

 of oxygen in the soil atmosphere for the maintenance of aerobic respiration. 

 When the supply of atmospheric oxygen to the roots is cut off by inundation 

 of the soil, anaerobic respiration replaces aerobic respiration. Possible causes 

 of the detrimental effect of continued anaerobic respiration on plant cells 

 will be considered later. 



]\Iost of the other known examples of anaerobic respiration in plants result 

 from structural features of plant organs which prevent ready access of oxygen 

 to interior tissues. For example, a number of species produce seeds with coats 

 which are only slightly permeable to oxygen. During the earlier stages of the 

 germination of such seeds, before the coats are ruptured, respiration is largely 

 of the anaerobic type. The best known example of this is in pea seeds, which 

 in the early stages of germination evolve a volume of carbon dioxide three or 

 four times as great as the volume of oxygen absorbed. Similarly the respira- 

 tion occurring in corn grains, oat grains (especially if the glumes are left 

 intact) and sunflower fruits during the early stages of germination is largely 

 anaerobic (Frietinger, 1927). A similar condition probably obtains in many 

 other seeds and dry fruits. 



Anaerobic respiration occurs naturally in some fleshy fruits. The "skin" 

 of some fruits, of which the grape is the most familiar example, is relatively 

 impermeable to oxygen, hence some anaerobic respiration undoubtedly occurs 

 in such organs. Somewhat similar conditions prevail in the ripening fruits of 

 the Japanese persimmon. 



It has been rather generally supposed that the interior tissues of most 



