PREFACE. Vll 



known facts, for which I am mainly indebted to general 

 treatises, monographs and shorter papers, I am constrained 

 to acknowledge for Part IV my special indebtedness to 

 Warming's Lehrbuch der okologischen Pflanzengeographie and 

 Ludwig's Lehrbuch der Pjlanzenbiologie. 



C. R. B. 

 University of Chicago. 



