120 Quantitative Analysis of Plant Growth 



the time relation of these points and thus the time relations of the maxima 

 on the curve. In extreme cases the environmental factors may so far 

 affect morphological differentiation as to cause coincidence of the 

 maxima. 



External conditions, in addition to causing modification in this way 

 in the general form of the growth-rate curve, must directly affect the 

 absolute value of the growth-rate, but an analysis of these curves and 

 attempts to correlate still smaller fluctuations in these curves from year 

 to year with external conditions have not yielded any definite results. 

 We shall return to the subject of the effect of external conditions when 

 dealing later with another form of expressing growth-rate, namely in- 

 crease in dry-weight per unit leaf-area per unit time. 



In a future chapter we propose to compare the relative growth-rate 

 curves of other annual plants with those for maize which have been 

 dealt with above. 



Average Growth-Rate. 



A full consideration of all the data presented here will show the 

 extraordinary difficulty of finding any valid basis for comparing plants 

 such as maize by means of their average growth-rate whether the 

 average is taken over the whole life-cycle, which is of varying length, 

 or whether arbitrary periods of shorter duration are taken. It is par- 

 ticularly misleading to compare the average growth-rate for one period 

 of one plant with a different period of another plant. For example, a 

 comparison of any two plants by means of their average growth-rate 

 over a period such as six weeks would be favourable to one, whereas 

 a comparison over say 12 weeks might be favourable to the other. 



In a subsequent chapter dealing with the question of growth-rate in 

 relation to yield, this point will receive detailed consideration. 



Summary. 



The series of articles of which this is the first instalment, constitutes 

 an attempt to formulate methods for the quantitative analysis of plant 

 growth and to apply these methods to data which have been lying 

 dormant in the literature for 40 years. 



In the present chapter the relative growth-rate curve, which is the 

 weekly percentage increase in dry- weight plotted against time, and also 

 the leaf-area ratio curve, that is, the leaf-area in sq. cms. per g. plotted 

 against time, have been employed. And as a typical example of an 



