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



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metre of leaf per week, taking as the leaf-area the average of the areas at 

 the beginning and at the end of the week 1 . 



Before studying these values for Unit Leaf Rate in detail let us con- 

 sider the possible ideal forms of the Unit Leaf Rate curve. Neglecting 

 the possible effects of environmental factors, if no change occurs in the 

 values of the assimilation and respiration per unit leaf-area the Unit 

 Leaf Rate curve will be a line parallel to the time axis. With regard to 

 assimilation, the activity of the leaves is by no means uniform. The 



A. Growth-Rate Curve. B. Leaf-Area Ratio Curve. 

 Fig. 1. 



activity of the seedling leaves is smaller than that of the more mature 

 leaves whether carbon dioxide, light, or temperature be limiting, a con- 

 clusion which we drew as the result of the analysis of Kreusler's data in 

 the previous chapter, and which has been demonstrated experimentally 

 for some plants by Irving (10) and by Briggs (4) 2 . This change in the 



1 In the appendix to this paper we have put forward two methods of calculating Unit 

 Leaf Rate. Except during the first few weeks it is immaterial which method of calcula- 

 tion is used. During this early period the increase in leaf-area is most probably inter- 

 mediate between linear and exponential, as an inspection of the figures for leaf-area 

 indicate, and consequently the actual values of the Unit Leaf Rate are intermediate 

 between those calculated by the two methods. The conclusions drawn in this paper are 

 not affected by the method of calculating L T nit Leaf Rate, and hence the simpler method 

 is used. The values for Unit Leaf Rate for the year 1877 run as follows: 6-9, 5-6, 5-1, 3-2, 5-8 

 on the exponential basis and 01. 5*1, 4-9, 3-1. 5-7 on the linear basis. 



2 The question of changes in assimilatory activity of the leaf throughout its develop- 

 ment, and of young leaves throughout the development of the plant requires more thorough 

 investigation than it has at present received. 



