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



Inspection of Table VI shows that, on the twenty-second day from germina- 

 tion, the value of the respiratory index of the young plant with three or four 

 pairs of leaves was about 3 ; on the fifty-fourth day from germination, the 

 values for the second, third, and fourth pairs of leaves (from the bottom of 

 the plant) were C'89, 0'89, and 1"0 respectively. On the ll2th day from 

 germination, on which date the three lowermost pairs of leaves had fallen, 

 the value of the respiratory index of those remaining on the lower half of 

 the stem was only 0'27. 



A similar decrease has been observed in the case of the flowers, whether 

 the main inflorescence alone or whether all the flowers of the plant be 

 considered. 



We come finally to the results obtained with the actively growing stem- 

 apex. The case here has the peculiar interest that we are dealing with a 

 tissue which, unlike the plant as a whole and the parts previously mentioned, 

 is not complicated by increasing differentiation with age, but is still mainly 

 meristematic and might therefore be expected to retain its original respiratory 

 activity. The respiratory index at the stem-apex is not, as might be expected, 

 constant throughout the life of the plant, but falls continuously as the age of 

 the plant increases. The stem-apex may be regarded as giving us the initial 

 respiratory index of each leaf as it appears ; that is, approximately the 

 respiratory index of the meristematic tissue. 



In Table VI it is seen that on the twenty-second day from germination the 

 value of the respiratory index of the top cluster of leaves (in this case the 

 entire seedling plant) was 3. On the fiftieth day from germination the value 

 for the respiratory index of the top cluster of leaves was 2-56, whilst on the 

 fifty-ninth day from germination it had fallen to 1"72. After this date the 

 inflorescence appeared, but, as will be seen from the same Table, on the fifty- 

 third day from germination the value for the respiratory index of a top cluster 

 of leaves without visible flower rudiments was the same (1"80) as that for a 

 top cluster of leaves from another plant in which the young inflorescence had 

 ■developed. 



In concluding this section we may consider the decrease with age in the 

 respiratory index of the whole plant with a view to separating the component 

 factors which we have until now grouped together as the " internal " factor. 

 It will be seen from Table IV that during the first fifty-nine days from 

 germination, after which period the inflorescence appeared, the greatest fall 

 in the value of the respiratory index occurs in the case of the stem, whereas 

 the fall is smallest in the case of the stem-apex. If we assume that the 

 value for the respiratory index of the stem, leaves, and stem-apex is the same 

 just after germination — the fact that the respiratory index of the entire plant 



