THE RESPIRATORY RATIO 51 7 



As indicated by the results shown in this table the minimum light require- 

 ment of species which ecological observations have shown can exist only in 

 well lighted habitats, such as some of the pines, is considerably in excess of 

 the requirement for such shade tolerant species as hemlock, beech, and sugar 

 maple. The maintenance of a species at the light intensity of the compensa- 

 tion point will not, however, permit its survival under natural conditions. In 

 the first place light is available in the natural habitats of plants for only part 

 of each day; hence there is no photosynthesis which compensates for night 

 respiration. In the second place, if, as is the usual practice, the compensation 

 point is measured only for the leaves or aerial organs of the plant, no allow- 

 ance is made for respiration of the roots or other non-green organs. Further- 

 more, no plant can survive indefinitely without the consumption of some food 

 in assimilation, and at the compensation point no food would be available 

 which could be used in this process. Hence the actual minimum light in- 

 tensities at which these species could survive in nature would necessarily be 

 somewhat greater than those indicated in Table 51. However, the order of 

 their minimum light requirements under natural conditions probably would 

 not differ greatly from that found in this investigation. 



The Respiratory Ratio. — The ratio of the volume of CO2 released to 

 the volume of Oo absorbed in the respiratory process is termed the respiratory 

 ratio or quotient. When complete oxidation of a hexose sugar occurs, as 

 already pointed out: 



CO2 



O2 



= I 



The respiratory ratio for any plant or plant part can be determined by making 

 parallel measurements of the rates of carbon dioxide release and oxygen con- 

 sumption. 



The respiratory ratio of germinating seeds in which the accumulated foods 

 are principally in the form of carbohydrates is invariably found to be ap- 

 proximately one as long as oxygen has free access to such seeds. This is true, 

 for example, of the germinating grains of practically all of the cereals (wheat, 

 maize, oats, etc.). Similarly the respiratory ratios for the leaves of many 

 species of plants have been found to be in the neighborhood of one (Table 52) 

 and flowers usually have a respiratory ratio of approximately one (Pringsheim, 



1935)- 



Unlike the photosynthetic ratio, the respiratory ratio of plant tissues is 



by no means always equal to unity, although most commonly this value is 



approximated. The proportion of the volume of carbon dioxide evolved to 



the volume of oxygen absorbed may vary greatly from a unit value, depend- 



