| 其他書名 | A Thesis Submitted to the Faculty of the Graduate School of the University of Minnesota, in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy, June, 1931 |
| 出版 | Department of Agriculture, 1932 |
| URL | http://books.google.com.hk/books?id=f9RBAAAAIAAJ&hl=&source=gbs_api |