RESPIRATORY INDEX 75 



temperature, food, facility of gaseous exchange and circulation 

 in the plant, and so on, are favourable. Considering the plant 

 as a whole, Bonnier and Mangin * recognize two respiratory 

 maxima in its seasonal development, the first at germination, 

 or on the unfolding of the leaf buds, and the second at the 

 opening of the flower buds. Ruby f found that the intensity 

 of respiration was greater in leaves of young than of old 

 plants ; thus the amount of carbon dioxide evolved per hour 

 per gram of fresh weight of leaves from trees one year old, three 

 years old, and many years old was respectively '2OO c.c., "150 

 c.c., and *ico c.c. In all cases the growth period showed a one- 

 and-a-half to two-fold increase in respiration as compared with 

 the non-growing periods. Nicolas J compared the respiration 

 of the vegetative parts of annual, bienniel, and perenniel plants 

 and found that leaves and portions of stems of the same branch 

 varied according to their age, those from the apical regions 

 showing a three to seven-fold intensity of respiration as compared 

 with similar structures from the basal parts. 



These observations have been confirmed and extended by 

 Kidd, West, and Briggs who have studied the respiration of 

 Helianthus annuus both in the laboratory and in the field. They 

 point out that the factors which may affect the rate of respira- 

 tion per unit of dry weight of tissue are the concentration of 

 the respirable material, the concentration of oxygen, the tem- 

 perature, and the effective amount of respiring cell matter per 

 unit of dry weight. This last is the " internal " factor, the re- 

 sultant of many factors, none of which as yet fully understood 

 and some of which probably not yet formulated. The internal 

 factor can only be accurately measured when the other factors 

 are not conditioning respiration. For purposes of measuring 

 its effect, Kidd, West, and Briggs employ a respiratory index 

 which is the respiration, measured by the rate of carbon 

 dioxide produced, per gram of dry weight at 10 C. when the 

 amount of respirable material is not limiting and when the 

 external concentration of oxygen is that of the atmosphere. 

 From a large number of observations they conclude that the 



* Bonnier and Mangin : " Ann. Sci. Nat. Dot.," 1885, 5, 315. 



fRuby: W., 1917, 20, r. 



Nicolas: " Rev. g6n. Bot.," 1918, 30, 209. 



Kidd, West, and Briggs: " Proc. Roy. Soc.," Lond., B, 1921, 92, 368. 



