G. E. Briggs, F. Kidd, and 0. West 115 



subtracted from gain due to assimilation, etc. Consequently it is obvious 

 that changes in the rate of respiration per unit dry-weight of sufficient 

 magnitude to affect the rate of growth to the extent observed are in- 

 conceivable. We must turn therefore to the changes in the rate of 

 assimilation per unit dry -weight in order to account for the main rise 

 and fall which characterises the growth -rate curve. Brief consideration 

 will show that the rate of assimilation per unit dry-weight is most 

 probably a function of the amount of leaf-area per unit dry-weight 

 mainly, and it is interesting to enquire therefore to what extent changes 

 in leaf-area per unit dry-weight correspond with those in the rate of 

 growth. The values of the ratio of leaf -area to dry- weight throughout 

 the life-cycle of the plant can be calculated from Kreusler's data, and 

 when these values are plotted against time there appears a striking 

 similarity between this curve and the growth-rate curve (see Figs. 

 4, 5, 6 and 7). 



From this we may conclude, therefore, that the main rise and fall 

 shown by the growth-rate curve is merely an expression of the rise and 

 fall in the ratio of leaf-area to dry-weight. 



To return to the question of the assimilation of the young leaves on 

 their first appearance, an inspection of Figs. 4, 5, 6 and 7 will show that 

 at this stage the ratio of the ordinate of the growth-rate to that of the 

 leaf-area curve (which ratio is really a measure of the increase in dry- 

 weight per unit leaf-area) is a negative or very small quantity compared 

 with the ratio during the main period of high relative rate of growth. 

 This fact strongly suggests that the assimilatory power of the young 

 leaves for some time after their first appearance is negligibly small. It 

 is interesting to find that this inference which is drawn from an analysis 

 of plant growth, as presented in this paper, is corroborated by direct 

 experimentation on the assimilatory power of young leaves (Irving (7) 

 and Briggs (2)) x . 



Another point of interest which arises from a comparison of the 

 leaf-area ratio with the growth-rate curve is that, while the growth- 

 rate curve exhibits one or more subsidiary maxima in the falling phase, 

 the leaf-area ratio curve on the other hand falls uninterruptedly. 



With regard to these subsidiary maxima exhibited by the growth- 

 rate curve there is a significant correlation between the times of their 

 occurrence and the recorded times of the first appearance of the male 

 and female flowers (see Figs. 4-8). Results obtained with maize by 

 Morgen(i8) and Osswald(iO) who worked in conjunction with Kreusler, 

 1 Also unpublished results for maize. 



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