MEXICAN WHOKLED MILKWEED AS A POISON'OUS PLAXT. 3 



Fleming (1920) gives the dosage by which experimental calves and 

 sheep were poisoned. 



Fleming and co-authors (1920) give a somewhat extended account 

 of the plant, with a report of feeding experiments on cattle and sheep. 



It seems probable from present investigations that man}^ of the 

 cases of poisoning attributed to A. speciosa, A. eriocarpa, and ^4. 

 fremonti were really due to A. mexicana. No detailed experimental 

 work, however, has been done on ^1. eriocarpa or A. fremonti. 



DESCRIPTION OF ASCLEPIAS MEXICANA.' 



The following is a technical description of the plant, Asclepixis 

 mexicana (see PI. I), known as the Mexican whorled milkweed, also 

 as the narrow-leaf milkweed. The stems are erect, single or several, 

 often branching at base, woody at base, 1 to 6 feet high; main root 

 horizontal, branching, producing adventitious buds; stems glabrous 

 below, slightly short, hairy, and branching above; the leaves in 

 whorls, 2 to 6, sometimes in axillary bundles, linear to narrowly 

 lanceolate, short petioled, 2 to 6 inches long, 2 to G lines wide, often 

 folding together, smooth, acute at both ends; flowers usually in 

 corymbose terminal umbels, many-Jflowered, peduncles much shorter 

 than leaves, and 2 to 3 times as long as the pedicels, which are sur- 

 rounded by numerous narrow bracts; corj^mb and flowers short, soft, 

 hairy; flowers greenish-white, sometimes purplish; calyx and corolla 

 turned back after opening; calyx five-cut; corolla deeply five-cleft; 

 crown of 5 hoods, hoods erect, open, containing a linear horn 2 or 3 

 times as long as the hood, and surrounding the column (united 

 stamens) in which are the 2 ovaries, with slender styles, which unite 

 to the single disklike stigma, the anthers adhering to the stigma and 

 opening on the outside, thus compelling the use of insects or some 

 other outside agency to transport the waxy mass of pollen to the 

 stigma disk; pods from 2 to 3 inches long, smooth, narrow, splitting 

 on the sides, seeds flat, reddish-brown, with a tuft of long hau's at 

 summit. It flowers from June to August. 



DISTRIBUTION AND HABITS OF ASCLEPIAS MEXICANA. 



The plant ranges from Mexico northward tlu'ough California, west- 

 ern Nevada, and southern Washington to eastern Idaho, as shown 

 in figure 1. 



For central California Jepson (1911) says, ''Forming patches in 

 dry ground, common and widely distributed in barren valley fields." 

 It is also a foothill species; in speaking of Nevada M. E. Jones, in an 

 impublished note, states that it occurs in juniper and oak zones. For 

 the range from northern California to Washington, Howell tells us that 



■ The descript ion of the plant and the account of its distribution were prepared by W. W. Eggleston, 



of the Bureau of riant Industry. 



