Although it is commonly acknowledged that we owe to Blackman the first clear statement of the mathematical principles underlying the law of exponential growth, his ‘efficiency index’ has had an extraordinary history of criticism and rejection. The efficiency index is, of course, none other than the relative growth rate – a concept which has always been eminently respectable. No doubt the nature of the analogy – that of compound interest – and reference to it as a physiological constant are the bases of the misunderstandings. His earliest critics were Kidd et al. (1920) and they were effectively answered by Blackman (1920) in the same volume of the New Phytologist. Of special interest is the contention of Kidd et al. that the only way in which plants can be compared is by the comparison of the whole series of efficiency indices throughout their life-cycles.