THE MOST IMPORTANT SPECIES. 27 



straight, not erect, but spreading or ascending, closely covered with 

 matted hairs like wool; leaves 4 to 8 pairs on each stem, ovate or 

 oblong, acuminate, 2.5 to 5 inches long, 1.5 to 3 inches wide, covered 

 like the stems with matted white hairs which are somewhat deciduous 

 as the leaves mature; flowers appearing from March to early July, 

 greenish or dull purplish, in rounded clusters (umbels), the clusters 

 pendent from the axils of the upper leaves and ends of the stems, 

 differing from all of the other milkweeds here described in having 

 neither horn nor crest to the hoods; pods on bent stalks, pendent or 

 erect, elongated ovoid, about 4 inches long, white with a dense wool- 

 like tomentum, maturing from late July into the autumn. 



References. — Torrey, Bot. Mex. Bound., 160, pi. 44, 1859 (as Acerates tomentosa) . Gray, , 

 Syn. Fl., 21; 100, 1878 (as Gomphocarpus tomentosus). Hall, Bot. Gaz., 31: 389, 1901. 



Distribution and ecology. — This milkweed grows only in western 

 California, from the latitude of San Francisco south to the Mexican 

 border and on the westerly slope of the Sierra Nevada from Mariposa 

 County to Tehachepi Pass. It grows scatteringly on the inner Coast 

 Ranges, where it reaches its northern limit on Mount Diablo, and is 

 common only in southern California. The largest quantities have 

 been found on the foothill slopes along Lytle Creek and Cajon Pass; 

 on sandy washes and slopes from Corona to Elsinore and Temecula; 

 and again on the foothills of San Gorgonio Pass north of Banning. 

 It grows also on the desert area, as, for example, near Victorville, 

 but it does not range far from the surrounding mountains. Although 

 an abundance of material for experimental purposes may be obtained 

 from these locahties, the species is nowhere truly abundant; that is, 

 it does not form pure stands over large areas. 



The ecologic requirements of Asclepias calif ornica include a rather 

 light, somewhat sandy soil, a very moderate amount of moisture, and 

 this only during the early spring, and warm, clear days during the 

 growing-period. The total annual precipitation at some of the 

 stations mentioned above does not exceed 16 inches, most of this com- 

 ing from January to April. The temperature falls to about 20** F. 

 in winter and rises to about 110° F. in summer. The plants are 

 usually surrounded by a sparse growth of grasses and other herbs, 

 but these are not sufficiently high to yield shade. Sometimes the 

 milkweeds are partially shaded by shrubs, but the species grows only 

 along the edges of the chaparral formation and does not creep up 

 under the shrubs for protection. 



Judging from conditions under which the plants grow wild, it would 

 seem that Asclepias calif ornica could be brought under cultivation on 

 exceedingly poor land. A light, well-drained soil should be selected 

 and a location where winter temperatures do not fall much below 20°. 

 High summer temperatures are quite certainly desirable, since the 



