vill PREFACE TO THE SIXTH EDITION 
new material on plasmodesma, waxes, plant hormones and 
vitamins. 
A new chapter has been added on “Genetics and Evolution” 
in keeping with the needs of some of the courses in botany. 
The chapters on “The Microscope’ and on “Histological 
Technique” have been transferred from their former positions 
in the main body of the text to the back of the book where they 
now appear as Appendix I and Appendix II respectively. New 
material has been incorporated under both of these captions | 
including modern methods of fixation, staining, imbedding and — 
mounting plant materials for microscopic examination. & 
The chapter on “Plant Tissues” has been lengthened and | 
improved by the introduction of much new material on the 
structure and physiology of meristems, endodermis, tracheary 
tissue, laticiferous tissue, xylem, phloem, the stele, bundles, 
leaf and branch traces, gaps and nectaries. 
The histological and physiological portions of the various 
chapters dealing with Plant Organs have also been augmented i 
with considerable new text and illustrative material. pe 
The Glossary has been retained and somewhat enlarged as 
has also the Classified List of Reference Works. A complete 
Index is also provided, making the data desired by the reader 
easily found. s 
Many new drawings, photographs and photomicrographs 
have been added in this revision which should prove helpful to 
the student. A considerable number of these are original; 
many have been borrowed. ae 
The writer desires to express his gratitude to the various 
publishers and authors from whom permission to use a number 
of the cuts used in this edition was cheerfully granted. Acknowl- 
edgment for these is given under the respective cuts in th 
text. He is particularly indebted to Dr. A. F. Sievers and the 
Bureau of Plant Industry for the loan of a number of prints on 
medicinal plants from which cuts were prepared, to Instructor. 
Richard W. St. Clair of the Massachusetts College of Pharmac 
for valued assistance in photographing many of the original 
figures, to Dr. F. J. Bacon of the Biology Department of Western 
Reserve University who furnished photographs on Zamia plan 
