220 Relations bet ween Groioth and Enclronnient 



Of the factors which affect the relative rate of increase of growth age, 

 maxiniiim and minimum temperatures and hours of sunshine can be 

 investigated from the available data. The relation between age and rate 

 of increase over the whole life is far from linear, so that the simple 

 formulae of partial correlation cannot be applied, but the series of mean 

 relative rates of increase may be divided into two satisfactorily linear 

 series by separating the young plants (mean age up to four weeks old) from 

 the old plants (mean age over four weeks). During the first period the 

 mean efficiency indices rise to a maximum and during the second they 

 fall from the maximum. This result bears out the observations that growth 

 is divided into two distinct periods and that the tendency is for the 

 maximum rate of growth to be attained at the beginning of the second 

 period and to fall off afterwards. 



When allowance is thus made for age the effect of environment upon 

 the young and old plants must be considered separately. 



The actual correlations between relative rates of increase (efficiency 

 index) and the four other variates, age and the three' environmental 

 measurements, are 



Youn.fr plants Old plants 



Age +-699ozh-063 --SigSiOeo 



Mean maximum temperature + -4444 ± -097 + -3907 ± -063 



Mean minimum temperature + -3958 ± -104 - -0588 ± -074 



Bright sunshine -•0132±-123 +ol77i:-056 



The quantities measure the extent to which the variables concerned 

 are actually associated; growth in young plants is thus associated with 

 relatively warm days and nights but not significantly with sunshine, 

 in the older plants it is associated strongly with sunshine and warm days 

 but not significantly with the night tem])eratures. 



The combined eft'ect of the independent variation of these four 

 quantities may be expressed by regression formulae, the coefficients of 

 which can be calculated from the correlations. If A stand for the age 

 measured from a mean value of 2-5 weeks and 10 weeks for young and 

 old plants respectively, C the mean maximum temperature measured 

 from 07° F., D the mean minimum temperature measured from 46° F. 

 and E the hours of bright sunshine in excess of 21 per week, then the 

 relative rate of increase of young plants has the regression formula 



•03 + 1-9544^ -I- •3586C- -OlOOZ) - •15.51A\ 



while for the old plants it is 



2-91 - -2121^ + -07515(7- -23202) + -OGlO/i;. 



