Quantitative Analysis of the Groivth 0/ Helianthus annuus. 379 



does not change appreciably for a period extending over more than three 

 weeks from germination being good evidence that this is the case — we find 

 that the fall in the respiratory index during this period of the stem, leaves, 

 and stem-apex is 89, 56, and 43 per cent, of the original value respectively. 

 The fall in the case of the stem and leaves must be partly due to the 

 increasing proportion of definitely non-living tissue, such as mechanical and 

 water-conducting tissues, but this cannot be the explanation of the fall in the 

 respiratory index of the stem-apex. The fact that the respiratory index of 

 the youngest leaves falls with the age of the plant is interesting and calls for 

 an explanation in itself. On our postulates the effective amount of respiring 

 cell-matter per gramme dry-weight in the young leaves becomes smaller as 

 the age of the plant increases. As to whether this is due to a decreasing 

 amount of protoplasm or respiring enzymes per gramme dry-weight, or to an 

 increasing amount of some inhibitor, remains an interesting question, to be 

 answered by further research. It is interesting to note, however^ that the 

 photosynthetic activity of the young leaves also falls off with increasing age 

 of the plant.* A knowledge of the protoplasmic nitrogen-content of the 

 whole plant and of its parts, particularly that of the young leaves, would 

 possibly throw light on the causes underlying the fall with age in the 

 respiratory index. 



6. Respiration oj Uncut Plants in the Field. 



The following experiments wth uncut plants in the field are of interest in 

 that they confirm the results already described, which were obtained with 

 cut plants in the laboratory. On five occasions, scattered throughout the 

 life-cycle, the respiration of an uncut plant was determined in situ in the 

 field. The leaves of the plants were drawn up to the stem and secured with 

 string ; an airtight fabric covering was then placed over the plant. This 

 covering was made airtight at the base by means of a waxed joint holding 

 water. The top of the covering was tied and also sealed with wax. The 

 inlet was a long narrow tube at the top, the outlet being at the bottom. A 

 current of air was drawn over the plant and thence through a gas meter. 

 The carbon dioxide was absorbed by caustic soda in a couple of Eeiset towers. 

 Allowance was made for the carbon dioxide present in the air at the rate of 3 c.c. 

 per 10 litres. The physiological mean temperature was taken and the 

 respiration results were reduced to 10° C. by means of the temperature- 

 respiration relation previously established. The results are presented in 

 Table YII, in which are also recorded the respiratory indices for the dates in 

 question. 



* Unpublished work by G. E. Briggs. 



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