16 BULLETIN 969, TJ. S. DEPARTMENT OF AGRICULTURE. 



the ratio of 1 to 6. There is an apparent anomaly in the dosage of 

 the stems in that the much more poisonous A. galioides requires a 

 sHghtly larger dose than A. mexicana. This may be explained in part 

 by the fact that the stems of both plants are only slightly toxic, but 

 there is also some reason for thinking that the chemical study of 

 these plants, the results of which will be published later, will offer an 

 adequate explanation. 



ANIMALS AFFECTED. 



The experiments reported in this bulletin were on sheep only. 



Fleming and his co-authors (1920) have shown that cattle are poisoned 



with a somewhat similar dosage. From the fact that A. mexicana 



produces effects strictly comparable with those caused by A. galioides 



and that it has been shown in a previous paper (U. S. Department of 



Agriculture Bulletin 800) that A. galioides poisons horses as well as 



sheep and cattle, it is fair to conclude that A. mexicana will have the 



same effect. 



SUMMARY. 



Asclepias mexicana (the Mexican whorled milkweed), a plant 

 growing on the Pacific slope, and especially in California and Nevada, 

 has long been considered a stock-poisoning plant, particularly dan- 

 gerous to sheep; but there has not been definite knowledge of the 

 symptoms produced by it or of its toxic dosage. 



It produces effects very similar to those produced by the other 

 whorled milkweeds, A. galioides, A. pumila, and A. verticillata var. 

 geyeri. 



In toxicity A. mexicana is about equal to A. pumila and about one- 

 fourth as toxic as A. galioides. The lethal dose is six times that of 

 A. galioides, but about one-half that of A. pumila. 



LITERATURE CITED. 



Chesnut, V. K. 1898. Preliminary catalogue of plants poisonous to stock. Annual 



report of the Bureau of Animal Industry. Pp. 387-420. 

 Fleming, C. E. 1920. Project 22 — Poisonous range plants. Rpt. Nev. Exp. Sta., 



p. 41. 

 Fleming, C. E., Peterson, N. F., Miller, M. R.., Vawter, L. R., Wright, L. H. 



1920. The narrow-leaved milkweed and the broad-leaved or showy milkweed. 



Bui. 99, Nevada Agricultural Experiment Station. 

 Hall, H. M., and Hall, C. C. 1912. A Yosemite flora. Pp. 189-190. 

 Hall, H. M., and Yates, H. S. 1915. Stcck-poisoning plants of California. Bui. 



249, Agricultural Experiment Station, Berkeley, Calif. 

 Howell, T. J. 1901. A flora of Northwest America. P. 441. 

 Jepson, Willis Linn. 1911. The flora of Western Middle California. P. 323. 

 Marsh, C. D wight, Clawson, A. B., Couch, J. F., and Eggleston, W. W. 1920. 



The whorled milkweed (Asclepias galioides) as a poisonous plant. L^. S. Department 

 of Agriculture, Bulletin 800. 

 Pammel, L. H. 1910. A manual of poisonous plants. Parti, p. 130. 



WASHINGTON : GOVEENMENT PKINTING OFFICE : 1921 



