5IO RESPIRATION 



physiologists. As thus employed with reference to the higher animals the 

 term is essentially synonymous with "breathing." 



However gaseous exchanges of the usual type are not invariably accom- 

 paniments of the process of respiration. Furthermore plants never "breathe" 

 in any fundamentally acceptable sense of the word, frequent popular and 

 semipopular statements to the contrary notwithstanding. Gases pass into 

 or out of plant organs by diffusion through the stomates, lenticels, or directly 

 through the peripheral walls of epidermal cells. Within the plant body gases 

 may be distributed by diffusion through the intercellular spaces or by cell to 

 cell diffusion as solutes. For these reasons plant physiologists use the term 

 respiration primarily to refer to the oxidation of foods in living cells resulting 

 in the release of energy. This is the one basic aspect of the process which 

 is common to virtually all living organisms. 



There is considerable evidence that hexose sugars are generally and per- 

 haps invariably the substrate which is oxidized in the cells of higher green 

 plants. When plant cells contain both carbohydrates and fats, the former 

 apparently are consumed first in respiration before any inroads are made upon 

 the accumulated fats. W^hen fatty seeds are allowed to germinate in contact 

 with a sugar solution it has been found that the sugar is oxidized first. Even 

 when fats serve as the respiratory substrate in plants they are apparently 

 first converted into simple sugars (in itself an oxidation process), which are 

 in turn oxidized to carbon dioxide and water. Utilization of proteins in 

 respiration apparently occurs only in tissues which have been depleted of 

 carbohydrates or fats. In starved leaves, for example, proteins are h3'drolyzed 

 to amino acids which are first oxidized to asparagine. Under more extreme 

 conditions oxidation of amino acids occurs, resulting in the release of ammonia 

 in the plant tissues (Chap. XXVI). Under such conditions it is believed 

 that the protoplasmic proteins may themselves be hydrolyzed and oxidized. 



In most plant organs the rate of respiration is relatively so slow that any 

 heat produced is rapidly dissipated into the environment and thus escapes 

 detection. The evolution of heat during the respiration of certain plant 

 organs can, however, be demonstrated under natural conditions. Temperatures 

 as much as 15° C. in excess of the surrounding atmosphere have been found 

 in the spadices of the skunk cabbage {Spathycma foetida), while the tem- 

 peratures within the spadices of Ariitn italiciim have been shown to sometimes 

 exceed atmospheric temperatures by as much as 36° C. This latter, how- 

 ever, must be regarded as an extreme example. Such a self-induced increase 

 in the temperature of a plant organ in itself results in an increase in the rate 

 of respiration and other metabolic processes occurring within that organ. 



Heat production during respiration can be demonstrated most easily by 



