12 EUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



III. ACKNOWLEDGMENTS. 



It is a pleasure to make grateful acknowledgment for assistance 

 received from a number of people who have been sufficiently interested 

 to gather and forward samples for analysis, and in some cases to supply 

 information of considerable value. The list of those who have con- 

 tributed in this manner mcludes the following: Professor E. B. Bab- 

 cock, University of Cahfornia; Mr. W. W. Eggleston, United States 

 Department of Agriculture; Mr. C. L. Forsling, Jornada Reserve, Las 

 Cruces, New Mexico; Mr. M. French Oilman, Banning, California; 

 Mr. C. Hildreth, Lmcoln, Nebraska; Mr. Ivan M. Johnston, University 

 of California; Professor W. L. Jepson, University of Cahfornia; Mr. 

 C. F. Korstian, United States Forest Service; Mr. J. V. G. Loftfield, 

 Tucson, Arizona; Dr. E. B. Payson, Missouri Botanical Garden; Mr. 

 Albert J. Perkms, Santa Ana, California; Mrs. C. P. Powell, Berkeley, 

 Cahfornia; Mr. Carl Purdy, Ukiah, Cahfornia; Miss Inez Sandusky, 

 Wichita Falls, Kansas; Mr. E. E. Schellenger, Riverside, Cahfornia; 

 Dr. Forrest Shreve, Carnegie Institution of Washington; Miss Laurene 

 Stevens, Lawrence, Kansas; Professor J. J. Thornber, University of 

 Arizona; Miss H. A. Walker, University of Cahfornia; Professor J. E. 

 Weaver, University of Nebraska. Professor E. C. McCarty, of the 

 Colorado Agricultm-al College, has generously made a number of 

 chemical examinations and assisted in other ways, and Mr. H. R. Reed, 

 of the United States Department of Agriculture, not only collected 

 numerous samples of Asclepias suhulata, but also carried on experi- 

 ments with this plant while he was stationed at the experiment farm at 

 Bard, Cahfornia. The studies have been carried out under the author- 

 ity of Dr. F. E. Clements, who has not only given his wholehearted 

 support, but has also collected many of the plants here considered. 



IV. METHODS EMPLOYED. 



Collection and Pbepahation of Samples. 



Most of the plants examined are perennial herbs. Samples of these 

 were gathered by severing the stems near the base, or at about the 

 level where a low-set mowing machine would cut them. The cut 

 ends were immediately plunged into fine dust to aid in the coagula- 

 tion of the latex and thus check the loss of rubber through bleeding of 

 the stems. Pulling was commonly resorted to in the case of annuals, 

 but the roots were discarded except in the few cases where the 

 analysis of the root is specifically indicated. This rejection of the roots 

 was practiced because of the almost total absence of rubber below the 

 soil-hne in latex plants and also because the roots would not be utUized 

 in the preparation of rubber for commercial purposes. 



A wooden drying-oven provided with screen-bottom trays was found 

 convenient for purposes of desiccation, the heat being supplied from 



