Vi PREFACE. 



is also important to know what is the systematic position of each 

 of the various plants whose leaves afford the material for study. 

 In a word, the student should not attempt to read the book 

 straight through from the beginning as if it were a novel. On the 

 contrary, he may begin with any one of the four parts as his main 

 subject ; but that part must be studied in close relation with the 

 other three parts, a procedure which is facilitated by the large 

 number of cross-references in the text. 



S. H. V. 

 July, 1898. 



