4 INTRODUCTION 



The best idea of the form and size of the individual cells is 

 gained from studying macerated material. This is readily obtained 

 by placing small pieces of wood in a test-tube together with a 

 number of crystals of potassium chlorate, and adding enough 

 nitric acid to cover them well. After the wood has turned white 

 the solution should then be poured off and the material washed 

 thoroughly in water. This action may be hastened by warming. 

 It is then easy to remove a small portion of the mass to a slide 

 where it can be dissected with a couple of needles and studied 

 under the microscope. 



The writer desires to acknowledge his indebtedness to P.rof. 

 James W. Toumey for much of the data upon which this work is 

 based; to Mr. Clayton D. Mell for many helpful suggestions and 

 criticisms; and to Mr. Charles J. Heller for the loan of a set of 

 wood sections from which the photo-micrographs were made 

 by the writer at the Forest Products Laboratory, Madison, Wis- 

 consin. 



