24 o PHYSIOLOGY OF NUTRITION 



latent heat of water vapor). Rapidly transpiring leaves are generally a little cooler 

 than the surrounding air, even in direct sunlight. 



When there are large numbers of very active cells crowded into a small space, with 

 not too ready heat conduction to the surroundings, the heat developed by respiration 

 becomes evident, and the temperature of the tissue may be much higher than that of 

 the surroundings. The internal temperature of an Arum spadix with opening flowers 

 may be more than 2 5°C. higher than that of the surrounding air. A mass of germinat- 

 ing seeds or of opening leaf-buds may develop very high temperatures (7 to 20°C. 

 higher than those of the surrounding air), especially if outward heat transfer is arti- 

 ficially hindered, as by enclosing the seeds in a Dewar flask. In germinating seeds the 

 maximum rate of heat production occurs very early, just after germination begins, and 

 this rate becomes lower as the seedlings develop. The highest rates of heat production 

 appear to occur with the lowest values of the respiration ratio. 



The heat produced by respiration is generally in excess of the amount calculated 

 from the carbon dioxide given off or from the amount of oxygen absorbed, considering 

 that carbon and oxygen simply unite to form carbon dioxide. A part of the difference 

 is accounted for by considering the process as starting with carbohydrate and oxygen, 

 instead of with carbon and oxygen. Not all the energy set free by respiration appears 

 as heat; some of it disappears in the performance of the various kinds of work accom- 

 plished in the organism. 



8. Anaerobic, or Intramolecular, Respiration.— In active plant tissues that usually 

 require oxygen, anaerobic respiration continues for a time after the supply of oxygen 

 has been cut off. As has been said, this part of the respiration process gives rise to 

 incompletely oxidized carbon compounds, such as alcohols, acids, etc., as well as to 

 some carbon dioxide or water, or both. If kept too long without oxygen supply, 

 tissues finally die, being perhaps poisoned by the accumulation of incompletely oxidized 



products. 



9. Respiration Chromogens and Pigments.— Pro-chromogens appear to be common 

 in plant tissues. These are substances apparently of the nature of glucosides. They 

 are decomposed by the glucoside enzyme emulsin, with the formation of correspond- 

 ing chromogens. The latter seem to play an important role in aerobic respiration; they 

 apparently unite readily with free oxygen under the influence of oxidizing enzymes, 

 producing water and corresponding respiration pigments. These pigments then act as 

 acceptors of hydrogen, uniting with the hydrogen produced by the anaerobic phase of 

 respiration, and thus produce the corresponding chromogens once more. The pig- 

 ments act as carriers of hydrogen, taking it up as it is produced (thereby becoming 

 chromogens) and then delivering it to free oxygen, with the formation of water (and 

 the re-formation of the pigments) : pigment + hydrogen = chromogen; chromogen 

 -f- oxygen = water + pigment. 



10. Respiration Enzymes.— Plant tissues may be killed without destroying their 

 enzymes, and they may then continue to give off carbon dioxide and to absorb oxygen, 

 but in a somewhat different way from that exhibited by the living tissues. From 

 studies on the respiration of such tissues, Palladin and others suggest that the carbon 

 dioxide given off in aerobic respiration all arises from the anaerobic phase, while the 

 water formed is a product of the union of free oxygen with respiration chromogens, in 

 the aerobic phase. It is supposed that Sugar and water unite and form carbon dioxide 

 and hydrogen, that the hydrogen unites with respiration pigments and thus forms chro- 

 mogens, and that the chromogens are directly oxidized by free oxygen, forming water 

 and the pigments. According to this hypothesis, as long as an unsatisfied acceptor of 



