118 Quantitative Analysis of Plant Growth 



are given in Fig. 8. These results are for tops only, not for the entire 

 plant, including roots, as in the other cases. 



It is striking that when there is only one prominent subsidiary 

 maximum the male and female flowers appear together. These subsidiary 

 maxima cannot be correlated with recorded variations in any climatic 

 conditions and consequently it seems safe to conclude that they must 

 be due to internal changes. 



In endeavouring to explain these maxima and their correlation with 

 the appearance of the male and female flowers in terms of assimilation 

 and respiration there are two alternatives. The first is to suppose that 

 at the recorded time of the appearance of the flowers there is a temporary 

 increase in assimilation per unit leaf-area or a decrease in respiration 

 per unit dry-weight, or a temporary increase in salt absorption by the 

 roots. The other alternative is to suppose that during the early stages 

 of flower development, prior to the first record, the reverse conditions 

 obtain, in other words, that the minima immediately preceding the 

 record of the appearance of flowers is to be attributed to these reverse 

 conditions. Since it is a well-known fact that flower development is 

 accompanied by an increased respiratory activity and also since we have 

 no evidence that there is an alteration in assimilation per unit leaf-area 

 connected with flower-formation, the safest conclusion at present seems 

 to be that the minima are to be correlated with increased respiratory 

 activity at these periods. 



Plants grown at the same time under similar conditions show a 

 coincidence of the maxima (Fig. 2), but when we compare plants grown 

 at different times and under different conditions the incidence of the 

 maxima varies (Fig. 3). It appears likely therefore that the incidence 

 of the maxima depends upon external conditions. As attempts to correlate 

 the maxima with the environmental conditions obtaining at the time of 

 their incidence were unsuccessful, we have concluded that most probably 

 the time of the incidence of the maxima is determined by environmental 

 conditions obtaining at previous stages in the plant's development. 



Having now considered the whole of the growth-rate curve for maize 

 it appears on the basis of the data available that the general form of 

 the curve and the occurrence of its various maxima are controlled by 

 internal changes intercorrelated with morphological developments. The 

 points in morphological development which appear to be significant are 

 (1) the rise to a maximum and the subsequent fall in the leaf -area dry- 

 weight ratio, (2) the development of the male flowers, and (3) the develop- 

 ment of the female flowers. Environmental conditions may influence 



