72 STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



The increased rate of growth in the dark, which has been very 

 commonly observed, can also in a measure be explained on the basis 

 of the foregoing experiments. In the dark there is an accumulation 

 of amino-acids in most plant parts. Given an adequate suppl}'^ of 

 carbohydrates, stored in the stems and other parts of the plant, 

 an increase in amino-acids resulting from the absence of light would 

 cause an increased respiratory rate. The behavior of an entire 

 plant has been described in the first experiment, under table 8 and 

 figure 7. It is therefore suggested that the augmented respiratory 

 activity is at least one important factor contributing to the increased 

 rate of growth during periods of darkness. 



There are many points regarding the relation of carbohydrates 

 and amino-acids to light which require further study. These ques- 

 tions seem to us to be of great importance to the problems of photo- 

 synthesis and are now being subjected to investigation. 



It would, of course, have been highly desirable if the proteins as 

 well as the amino-acids could have been determined in the leaf 

 material. This, however, was impossible, largely because of the 

 relatively small amount of material remaining from the respiration 

 determinations and from the analyses for amino-acids and sugars. 

 In view of the fact that the leaves were kept in a nutrient solution 

 containing no nitrogen, it seems highly probable that the amino- 

 acid increase came from the breakdown of the proteins. That the 

 leaves contained ample proteinaceous material to supply the increase 

 in amino-acids becomes evident from the following analyses. A 

 typical dry-leaf material of Heliarithus contained 5.50 per cent of 

 nitrogen as NH2, determined by the Folin micro-Kjeldahl method. 

 The water extract yielded 0.119 per cent nitrogen as NH2 by the 

 Van Slyke method, representing free amino-acids, and 0.463 per 

 cent nitrogen as NH2 by the Folin micro-Kjeldahl method. Evi- 

 dently some nitrogenous material other than amino-acids is extracted 

 by means of water. This, however, does not affect the Van Slyke 

 results. Some idea of the amount of protein in the dry-leaf 

 material can be gained from the results of the hydrolysis with 20 

 per cent hydrochloric acid. This hydrolyzed material contains total 

 nitrogen as NH2 of 5.03 per cent by the FoUn micro-Kjeldahl method. 

 The discrepancy between this figure and the 5.50 per cent represent- 

 ing the total nitrogen of the material which had not been previously 

 treated with 20 per cent acid must represent the humin formation 

 resulting from the reaction of the amino-acids with the carbohydrate. 

 The hydrolyzed material yielded amino nitrogen by the Van Slyke 

 method of 2.88 per cent. Although the latter value is undoubtedly 

 a little low on account of the humin formation during hydrolysis, 

 it represents a considerable reserve of protein when compared with 

 the original amino nitrogen in the water extract of 0.119 per cent. 

 Thus the hydrolyzed material contains about twenty times more 



