VI PREFACE. 



The footnotes in Part I (which form an addition to 

 the original draft of the work) merely supply a rough 

 guide to some parts of the literature. Nor is any attempt 

 made to give a full account of the literature of the subjects 

 dealt with in Part II. The papers enumerated at the 

 head of each chapter are merely recommended as being 

 illustrative of the use, in actual research, of the methods 

 described. 



We gladly take this opportunity of expressing our 

 thanks to Mr F. F. Blackman, Demonstrator of Botany in 

 the University, for much valuable help in the arrange- 

 ment of the experiments in Part I. Also to the 

 Cambridge Scientific Instrument Company for the use of 

 the cliches for Figs. 25 and 26. 



Botanical Laboratoet, Cambridge. 

 August 19, 1894. 



