Preface • vii 



not because it is the first or most important of its kind but simply 

 because it provides a particularly good example of a problem under 

 discussion. The great preponderance of English-language refer- 

 ences is simply a concession to the convenience of both reader and 

 writer, and does not reflect the frequency or importance of publi- 

 cations in other languages. Fortunately for the English-speaking 

 world most of the work from other countries is reviewed, and much 

 is even reported, in English by the original workers themselves. 



I have adopted the following convention with regard to cita- 

 tions in the text. If a statement is followed simply by author (s) 

 and date, for example, Hamner (1940) , the paper cited has original 

 data on the point in question. Directions to see a paper, on the other 

 hand, indicate reviews or other discussions from which further 

 references may be obtained. All plants are referred to for the first 

 time by both common (if any) and scientific names. Thereafter, the 

 practice adopted is arbitrary, but the index can always be used to 

 establish one from the other. 



In summary, I have tried to treat the field in a manner not 

 quite like that to be expected from a technical review or article, but 

 in such a way that the previously uninformed reader will afterward 

 be able to read any of these with understanding and enjoyment; and 

 then, best of all, perhaps try his own hand at the game. 



W. S. H. 



New Haven, Connecticut 

 September, 1961 



