INTENSITY ill 



pared the respiration of the vegetative parts of annual, biennial, 

 and perennial 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. 



Kidd, West, and Briggs * studied the respiration of Helian- 

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

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

 piration per unit of dry weight of tissue are the concentration 

 of the respirable material, the concentration of oxygen, the 

 temperature, and the effective amount of respiring cell matter 

 per unit of dry weight. This last is the internal factor, the 

 resultant of many factors, none of which as yet fully under- 

 stood and some of which probably not yet formulated. The 

 internal factor can only be accurately measured when the other 

 factors arc not conditioning respiration. For purposes of 

 measuring its effect, Kidd, West, and Briggs employ a respira- 

 tory 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 con- 

 clude that the respiratory index of the entire plant continu- 

 ously declines with increasing age. For example, entire 

 plants 2 days from germination gave 3 mg. carbon dioxide 

 per gram of dry weight per hour, whilst plants 136 days from 

 germination yielded but -39 mg. carbon dioxide per gram of 

 dry weight per hour. A similar decrease in the respiratory 

 index is exhibited by the stem, leaves and flowers. In 

 the stem the value fell from -8 mg. on the 36th day from 

 germination to -08 mg. on the 136th day from germination ; 

 during the same period the measure for the leaves decreased 

 from 1-56 to -44 mg. The fact that the initial respiratory 

 index of successive leaves decreases with the age of the plant 

 indicates a respiratory decrement of the merismatic tissue 

 with age and from this it follows that the fall in the respiratory 

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



