162 TEXTBOOK OF BOTANY 



There are other species, however, that survive without access to the 

 atmosphere through large internal air passages. Willows, bitter pecan, 

 and overcup oak are examples. Algae and other continuously submerged 

 plants may grow where the amount of oxygen is very low. Of course, 

 during the day, when photosynthesis is occurring, they have an abun- 

 dance of oxvgen. 



The agricultural practice of draining soils is a common means of 

 supplying the roots with an adequate amount of oxygen. Coarse soils 

 are much better aerated than those of extremely fine texture. This fact is 

 often exemplified bv the differences in plants on such soils. The greater 

 root growth in the small crevices of soils is undoubtedlv related to the 

 better aeration there. Soaking the soil of potted house plants about once 

 each week not only assures more adequate watering but also results in a 

 fresh supply of air to the roots as the water disappears from the soil. 



The storage and shipping of fruits and bulbs introduce another situa- 

 tion in which a deficiency of oxygen may be detrimental. The cells in 

 these plant organs are still alive, and their well-being depends upon a 

 certain amount of respiration. If fruits are packed too closely, many of 

 the cells die of suffocation and become brown; the color and flavor of the 

 fruits that survive mav become less desirable. Dark and hollow centers 

 in potatoes are often the result of the death of cells bv suffocation, 

 because of high temperature and a lack of oxvgen in storage bins. The 

 need of further knowledge of the relation of respiration to the best 

 preservation of fruits and other plant organs in storage and shipping 

 accounts for some of the research that is being done on the best methods 

 of refrigeration and ventilation in storage houses and shipping cars. 

 Truckers commonly transport ripe fruits at night to avoid the su£Focation 

 of cells in the fruit that would result from the higher daytime tem- 

 peratures. 



When plant tissues are cut or otherwise injured, the cut or injured 

 surfaces are exposed to an increased supply of oxygen. The exposed 

 surface often becomes dark colored because of the oxidation of tannin 

 or other substances in the cells. In the preparation of dried fruits this 

 darkening is undesirable; either methods are used to prevent it, or the 

 finished product is subjected to a bleaching process. 



Water content effects. A deficiencv of water mav influence the rate 

 of respiration primarilv because cell processes are dependent upon the 

 reactive substances being in solution in water. This condition is most 

 common in dry seeds. For instance, the rate of respiration in wheat seeds 



