PREFACE. VU 



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 

 ^\'arming's Lehrhiich der iikologischen Fflanzengeographie and 

 Ludwig's Lehrbuch der Pflanzenbwlogie. 



C. R. B. 

 UNivERsrrv of Chicago. 



