Drift of Net Assimilation Rate in Plants

Abstract

IN reply to the letter from R. F. Williams¹, may I say that I much regret not having mentioned his paper² and that of Ballard and Petrie³ in my note on the effect of age on net assimilation and relative growth-rates in the cotton plant⁴, but I have just seen them for the first time. In stating that the absence of general trend of net assimilation rate up to time of flowering in my experiments was not proved, I was merely mentioning an inherent characteristic of the statistical test of significance. In testing the significance of a linear regression coefficient, the hypothesis is made that its true value is zero. This hypothesis may be disproved, at a given level of significance, in which case a significant general trend is shown, but it can never be finally proved. My results were clearly consistent with the hypothesis that the linear regression of net assimilation rate on time up to first flowering was zero, that is, that there was no general rise or fall, and it would seem, therefore, that to claim that my data confirmed the findings of Gregory⁵ for barley was justifiable. Furthermore, mzy results agreed with those obtained for cotton by Crowther⁶ in the Sudan, although no rigid statistical test was in the latter case applied.