THE MOST IMPORTANT SPECIES. 33 



ASCLEPIAS GALIOIDES (WhORLED MiLKWEED). 



Description. — An erect perennial herb, sometimes sligiitly woody at 

 the base, 1 to 3 feet high; roots erect and much branched just below 

 the surface, but these crowns connected below by horizontal root- 

 branches; stems few to numerous at a place, straight, unbranched, 

 green, and glabrous; leaves 20 to 60 on each stem, in whorls of 3 

 or 6 each or some of the lower only in pairs, linear, tapering to the 

 apex, 2 to 4 inches long, about 0.8 inch wide, green and smooth on 

 both sides; flowers appearing from June to August, dull white, small, 

 in small stalked umbels from the axils of the upper leaves ; pods strictly 

 erect, oYi straight stalks, fusiform, much narrowed above, 3 to 4 inches 

 long, scarcely 0.5 inch thick, glabrous and smooth, the seeds ripening 

 in August and September. 



/?e/erences.— Britton and Brown, 111., Fl., ed. 2: 32, fig. 3405, 1913. Marsh et al, U. S 

 Dept. Agr. Bull. 800, pis. 1 to 3, 1920. 



Distribution and ecology. — This is a species of the southern Rocky 

 Mountain region, from middle Utah and middle Colorado (Glenwood 

 Springs and valley of the Arkansas), south into Mexico and Central 

 America. It seems to be most abundant in Arizona and New Mexico. 

 Specific localities where it may be obtained in abundance include El 

 Paso in Texas and Flagstaff and Holbrook in Arizona. The natural 

 habitat of this plant is on the dry plains and foothills, often in sandy 

 soil. It is abundant on overgrazed areas, where it often replaces the 

 original grasses and leguminous plants, and also in fallow fields. It 

 responds quickly to the influence of cultivation or irrigation, as along 

 ditch-banks, and then makes a rank growth, spreading rapidly over 

 large areas. The downy seeds are carried both by wind and by irriga- 

 tion-water, while in cultivated fields the horizontal roots are broken 

 up and distributed, each piece giving rise to a new plant. When this 

 milkweed gets a start in neglected orchards it sometimes forms a solid 

 growth between the trees. Its poisonous nature has been described 

 by Marsh et al. (1920). 



Asclepias galioides has been much confused with a more easterly 

 species, namely, A. verticillata, a species which is sharply set off by 

 the whorls of numerous fibrous adventitious roots from the lower 

 nodes of the stem and by other characters, as recently emphasized 

 by Eggleston (in Marsh et al., 1920). It is, in reality, much more 

 closely related to A. mexicana, from which it is scarcely distinguish- 

 able, except by its usually narrower leaves. Also in the matter of its 

 rubber-content and in its ecologic requirements, A. galioides closely 

 resembles A. mexicana, and it is not unlikely that it should be treated 

 as a geographic variety of this more westerly species. 



Rubber-content. — The small number of analyses made of this 

 Bpecies does not warrant the drawing of definite conclusions as to the 



