X PREFACE 



I also have pleasure in thanking Dr C. S. Piper of the Waite 

 Agricultural Research Institute, University of Adelaide, for 

 permission to use the photographs reproduced in Figs. 4 and 12, 

 and my colleagues Dr K. W. Dent and Mr A. E. Roberts for the 

 preparation of the rest of the photographs in the book. 



Throughout the book the following contractions have been 

 used: 



ml. (millilitre) = one -thousandth of a litre. 



//g. (microgram) = one-millionth of a gram. 



A. (Angstrom unit) = one ten-thousandth of a millimetre. 



p. p.m. = parts per million. 



WALTER STILES 



Birmingham 

 28 October 1944 



