370 



Messrs. F. Kidd, C. West, and G. E. Briggs. 



respiration. Eecent work in this laboratory has shown that such is not the 

 case, but that the carbon dioxide output,* starting from low pressures of 

 oxygen in the atmosphere, increases with rising partial pressures, at first 

 rapidly and then more and more slowly until a maximum is reached, and then 

 decreases. The position of this maximum will presumably depend upon the 

 permeability of the tissue to oxygen, the internal concentration of oxygen 

 being the important factor. Although experiments have not been carried out 

 with Helianthus, the results obtained with similar tissues of other plants 

 indicate that an external concentration of 21 per cent, is sub-maximal, but 

 variations in the concentration in this region only slightly affect the rate of 

 carbon dioxide production. We have assumed that in using a constant 

 external concentration of oxygen we have been getting a close approximation 

 to a constant internal concentration, and therefore in view of what has been 

 said above, that the results will be comparable. 



With regard to the concentration of respirable material, if the rate of 

 respiration remains constant, other factors being constant, it is probably safe 

 to assume that the concentration of respirable material is in excess. Eespira- 

 tion expressed per gramme dry-weight per hour, when measured with the 

 respirable material in excess, with the external concentration of oxygen that 

 of the atmosphere, and with the temperature at 10° C, we propose to call the 

 respiratory index, and from what we have said above, the respiratory index 

 may be taken as a measure of the effective amount of respiring cell-matter 

 per gramme dry-weight, or in other words, a measure of the " internal " 

 factor.f 



3. Matericd and Methods. 



A detailed account of the plants used and of the manner in which they 

 were grown will appear in the further papers of this series. In the present 

 communication it will suffice to state that the variety of Helianthus anmms 

 used was Sutton's " Giant Yellow " and that the plants were grown in the 

 field under natural conditions of illumination and temperature and under 

 optimal conditions of spacing and soil humidity. The seeds were sown on 

 May 22, 1920, and the seedlings appeared above the surface on the 27th. 

 While the plants were small the output of carbon dioxide was determined by 

 absorption with baryta in Pettenkofer tubes, but later in the case of the 



* In the absence of any deeper knowledge of the process of respiration, the rate of 

 carbon dioxide production is employed as a measure of the rate of the process. 



t Palladin (7) has shown for seedlings that the respiration under constant external 

 conditions, when carbohydrate is in excess, is proportional to the fraction of protein 

 which is unattacked by gastric juice, thus suggesting that the amount of nuclein nitrogen 

 is a measure of the amount of respiring cell-matter. This suggests that the respiratory 

 indiex of the plant may be related to ita nuclein nitrogen-content. 



