PREFACE. 



This work was undertaken with a \'iew to sift- 

 ing the historical records of Ipomoea batatas and 

 furnish a working basis for future experimenta- 

 tion with its many varieties. It is now recognized 

 among botanists in general, and physiologists and 

 agriculturists in particular, that for obtaining reli- 

 able data, the variety and not the species must form 

 the basis of experiments. 



I wish especially to acknowledge my indebtedness 

 to Dr. John M. Macfarlane for general directions 

 and valuable suggestions. 



Herewith I present this work as a thesis offered to 

 the University of Pennsylvania in partial fulfilment 

 of the requirements for the degree of Doctor of 



Philosophy. 



B. H. A. Groth. 



Philadelphia, June, 1906. 



