34 STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



For the determination of amino-acids, 1 gram of the dry material 

 was heated for 3 hours with 25 c. c. of water on the boiling water-bath. 

 This was filtered and thoroughly washed with hot water. The 

 filtrate and washings were concentrated on the water-bath to 25 c. c. 

 It was found inadvisable to try to remove the small amount of soluble 

 protein from this extract, because any procedure designed to accom- 

 plish this also affected the amino-acid content, and this amount of 

 protein exerted an exceedingly slight influence on the amino-acid 

 determinations. These determinations were made by means of 

 the Van Slyke micro apparatus. Deleano^ stated that the nitrous- 

 acid method of determining amino groups was unreliable when there 

 were carbohydrates and nitrates present in the mixture. Special 

 tests made with the Van Slyke apparatus with pure amino-acids to 

 which nitrates and sugars were added did not confirm this statement 

 of Deleano. ' With some of the plant extracts there was considerable 

 frothing, but this was obviated by the use of a drop of caprylic 

 alcohol added to the reaction mixture in Van Slyke apparatus. 



The results of the respiration-rate determinations are reported 

 on the basis of the amount of carbon dioxid emitted per gram 

 of dry material. This is of course an arbitrary method and of only 

 relative value. However, there exists as yet no rational basis in 

 plant physiology of connecting the respiratory activity of vegetable 

 organisms, and the establishment of such a basis must await a 

 clearer understanding of the nature of the process itself. The 

 basis of fresh weight has the great disadvantage that the alterations 

 in water-content, which are easily effected in plants, very materially 

 change the results. The use of a surface standard, as has been done 

 in some work in animal physiology, has Uttle to recommend it in 

 work with plants. 



In the interpretation of analytical data obtained from living 

 material great care must be exercised to refrain from accepting the 

 apparent results as the true state of affairs. Very rarely is a single 

 chemical change unaccompanied by other changes which may run in 

 the same or opposite directions. Especially is this difficulty encoun- 

 tered in the interpretation of analytical data of leaves calculated 

 on the basis of percentage of original dry or fresh material. Thus 

 the increase in one component very often simply means the decrease 

 of another component which goes to make up the total mass of the 

 original material. 



In the tabulation of the experimental data some liberty was also 

 taken in expressing the time in decimals of hours instead of in hours 

 and minutes. However, this method facilitates greatly the work 

 of calculating and avoids a common source of error. 



J Deleano, N. Jahrb. wisa. Bot., 51, 552 (1921). 



