MEXICAN WHOKLED MILKWEED AS A POISONOUS PLANT. 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 many of the 
cases of poisoning attributed to A. speciosa, A. eriocarpa, and A. 
fremonti were really due to A. mexicana. No detailed experimental 
work, however, has been done on A. eriocarpa or A. fremonti. 
DESCRIPTION OF ASCLEPIAS MEXICANA. 1 
The following is a technical description of the plant, Asclepias 
mexicana (see PL 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 6 lines wide, often 
folding together, smooth, acute at both ends; flowers usually in 
corymbose terminal umbels, many-flowered, peduncles much shorter 
than leaves, and 2 to 3 times as long as the pedicels, which are sur- 
rounded by numerous narrow bracts; corymb 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 hairs at 
summit. It flowers from June to August. 
DISTRIBUTION AND HABITS OF ASCLEPIAS MEXICANA. 
The plant ranges from Mexico northward through 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 
unpublished note, states that it occurs in juniper and oak zones. For 
the range from northern California to Washington, Howell tells us that 
1 The description of the plant and the account of its distribution were prepared by W. W. Eggleston, 
of the Bureau of Plant Industry. 
