At the time of his death, Petrie had completed early drafts of two or three chapters of a book which was to have been called ‘The Developmental Physiology of Plants’, and the above quotation from Plato had been placed at the beginning of Chapter 2, ‘The Change in Dry Weight and Leaf Area, and First Steps in the Analysis of Growth Rate’. Unfortunately the book had not reached a stage from which it could have been completed by any of his colleagues, and we had to content ourselves with placing the quotation on the title page of a bound volume of Petrie's papers in plant physiology for the Library of the Waite Institute, Adelaide.

The second quotation is from one of Petrie's published papers, and epitomizes his thinking and general approach to the study of growth, though more extended statements along the same lines had in fact appeared a year earlier (Ballard and Petrie, 1936).