Iv PREFACE 



ment is made to the Pennsylvania State College for the 

 use of numerous negatives, and to Professors J. W. 

 Gregg and C. E. Myers of this institution for assistance 

 in making photographs ; to Professor L. C. Corbett and 

 Professor W. W. Tracy, of the United States Depart- 

 ment of Agriculture, for Figures 82, 84, 90, 93, 94, 95, 

 100, 102, 103, and 104; to H. B. Fullerton, of the Long 

 Island Railroad Experimental Stations, for Figures 50, 

 54, 73, 81 and 75; to Professor C. G. Woodbury, of the 

 Indiana Experiment Station, for Figures 87 and 88; to 

 Prof. H. H. Whetzel, of the Cornell Experiment Station, 

 for Figures 62 and 72 ; to Dr. W. L. Howard, of the Mis- 

 souri Experiment Station, for Figure 24; to Professor 

 John W. Lloyd, of the Illinois Experiment Station, for 

 Figure 89; to R. H. Garrahan, Kingston, Pa., for Fig- 

 ure 21 ; to H. R. Pennypacker, Phoenixville, Pa., for 

 Figure 78; to Lord & Burnham, for Figure 19; to the 

 Skinner Irrigation Co. for Figures 15, 16 and 17; to 

 Robert J. Walton, Hummelstown, Pa., for Figures 55 

 and 76; to F. J. Zuck, Erie, Pa., for Figure 56; and to 

 J. G. Curtiss, Rochester, N. Y., for Figures 69, 70 and 71. 



RALPH L. WATTS. 

 State College, Pa., 1911. 



