24 RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



favor, since this is conducive to ease of manipulation in harvesting. 

 The robust stems, even of wild plants, are sometimes 0.75 inch thick, 

 but they are not so firm as to render harvesting by machinery especially 

 difficult. The height of the stems, well-clothed to the summit with 

 leaves of fairly large size, indicate that the tonnage that could be 

 grown per acre is probably greater than in any other species, but unless 

 forms can be found in which the stems carry a larger percentage than 

 the chemical analyses usually show, it will be necessary to use only the 

 leaves, which constitute but 50 per cent of the total weight of the plant. 



Rubber-content. This milkweed has yielded the highest percentage 

 of rubber of all the species thus far tested. The most remarkable 

 records are those of plant No. 817, with 5.0 per cent in the leaves and 

 8.2 per cent in the stem; and No. 886, with 8.1 per cent in the leaves 

 and only a trace in the stem. The high percentage in the stem of No. 

 817 is unique and needs verification through the examination of a 

 larger series of plants. Yet the results seem trustworthy, since they 

 were checked by a second analysis, and both analyses were carried 

 out with the greatest care and in the same manner as the others. The 

 average of the 19 lots of leaves analyzed is 3.7 per cent, and there is 

 the unaccountable individual variation always encountered when a 

 large series of plants is analyzed. The encouraging feature is the 

 remarkably high percentage in a few cases, possibly indicating superior 

 strains suitable as a beginning in breeding experiments. 



ASCLEPIAS SYRIACA (COMMON MlLKWEED). 



Synonym: Asdepias cornuli. 



Description. Plant a stout, erect perennial herb, 3 to 6 and some- 

 times 7 feet high; roots probably spreading by deep horizontal branches; 

 stems usually several at a place, often very numerous and crowded so 

 as to form thickets of large extent, simple up to the flower-clusters, 

 straight, at first finely soft-hairy; leaves numerous to the top, usually 

 20 or more on each stem, opposite, lance-oblong or broadly elliptic, 

 5 to 8 inches long, 2 to 4 or rarely 6 inches wide, thick, pale, soon 

 smooth and glabrous above, but minutely downy beneath; flowers 

 dull purple to white, appearing from late June to September, many in 

 stalked umbels from the upper leaf-axils; pods on deflexed stalks, erect, 

 ovoid and acuminate, 3 to 5 inches long, 1 to 1.25 inches thick, gray 

 with a dense wool-like tomentum, with small soft spine-like projec- 

 tions, maturing in late August and September. 



References. Gray, Syn. FL, 2 1 :91, 1878 (as A. cornuti). Britton and Brown, 111. FL, 

 ed. 2:30, fig. 3398, 1913. 



Distribution and ecology. This is the most abundant milkweed in 

 the eastern United States and Canada, where it grows from New 

 Brunswick to North Carolina and west as far as Kansas and Saskatch- 



