114 Quantitative Analysis of Plant Growth 



It follows that the relative rate of growth at any time is almost the 

 same as the difference between the rates of assimilation and respira- 

 tion per 100 g. dry-weight at that time. 



We will now proceed to consider the curves. In following the curves 

 in Fig. 3 from the date of sowing, there is seen to be an initial phase 

 lasting for about three weeks during which the rate of growth is negative, 

 in other words, the plant is actually losing in weight 1 . This phase 

 of negative growth persists until a point in the development of the 

 plant is reached at which approximately four leaves have appeared. 

 During the time occupied by germination, before the appearance of these 

 leaves, the negative rate of growth is clearly to be attributed to a loss 

 of carbohydrate through respiration. The order of magnitude of the 

 loss in dry- weight through respiration in germinating seeds is 3 % to 6 % 

 of their dry- weight per day at 16° C. (Garreau(5)). In the latter part of 

 the period, where, despite the fact that the plant possesses from 1-4 leaves, 

 the negative rate of growth persists, it is obvious that any increase in 

 dry-weight due to assimilation is more than counter-balanced by a loss 

 in weight through respiration. Evidence obtained by an analysis of 

 Kreusler's data as to whether the leaves at this stage perform their 

 normal assimilatory function, or not, will be considered shortly. After 

 this initial phase there ensues a short period varying from 1-4 weeks 

 during which the rate of increase in dry-weight rises rapidly to its 

 maximum value, followed by a long period constituting the remainder^ 

 and larger part, of the life-cycle of the plant, throughout which the rate 

 of growth falls off more or less continuously. This falling part of the 

 curve, however, shows subsidiary maxima. 



The question arises of what kind of change in the plant this perfectly 

 definite type of curve in the main period of growth is an expression. It 

 is clear that this main rise and fall must be due to an increasing difference 

 between the rate of assimilation and the rate of respiration per unit dry- 

 weight in the first phase and to a decreasing difference in the second phase. 

 The order of magnitude of respiration in terms of dry matter consumed 

 per week during the main growth period of the plant is probably not 

 greater than 20 %-40 %, which is the order of magnitude of the loss 

 in dry-weight through respiration during germination. As against this 

 the actual percentage increase in dry-weight per week varies from % 

 to over 200 %, this being the balance when loss due to respiration is 



1 In the year 1875 the first dry-weight measurement was not taken until the end of 

 the third week. The average plotted in Fig. 3 gives no indication of the variations in the 

 individual weekly growth-rates. 



