THE MOST IMPORTANT SPECIES. 15 



V. THE MOST IMPORTANT SPECIES. 



Of the 64 species examined and found to contain rubber, there are 

 16 which seem sufficiently promising to warrant treatment in some 

 detail. These have been selected because of their comparatively high 

 rubber-content, this running from 1 to 8 per cent of the dry weight, at 

 least for mature leaves. Whether or not this is sufficient to encourage 

 experimental planting with a view to growing the rubber commercially 

 is a matter concerning which the writers are not competent to judge. 

 It is certain, however, that such percentages have not been approxi- 

 mated in any latex-bearing plant native to the United States that has 

 been heretofore examined, and since this fact may render them of 

 special interest to the rubber industry as well as to the botanist, the 

 following descriptive accounts have been prepared. 



The Genus Asclepias, or True Milkweeds. 



Since most of the species here discussed belong to the genus Asclepias, 

 a preliminary account of this group of plants will avoid repetition. 

 The genus comprises somewhat more than 100 species, and is best 

 represented in tropical and southern Africa. About 46 species are 

 native to the United States and Canada, and a number of these are 

 represented by additional varieties and forms of more or less importance. 

 All of the North American species are perennials, with deep roots, 

 which spread, in most cases at least, by horizontal branches which 

 give rise at intervals to vertical roots, these in turn dividing when 

 they reach the surface of the soil to form the crown from which new 

 stems arise. In some species this creeping of the roots is so extensive 

 that a single plant comes to occupy areas of large extent, covering 

 them with dense stands of leafy stems. Vegetative propagation by 

 portions of the horizontal roots is entirely feasible. The horizontal 

 root-branches have been almost universally mistaken for rootstocks, 

 but their true nature has been recently pointed out by Eggleston (in 

 Marsh et al., 1920). But in at least one species {A. suhulata) there 

 seems to be only a single taproot, without connection with other tap- 

 roots, but this needs verification. 



A few of the milkweeds have woody stems, but those here described 

 die down to near the base each autumn or winter, new shoots appear- 

 ing from the same root or stump .at the beginning of the following 

 growing-season, which usually follows the melting of the snows in 

 the colder regions or the first rains in the warmer and arid south- 

 western districts. When the stems are injured new ones spring up 

 to take their place. There can therefore be no doubt that if a crop 

 is once taken off for any purpose it will be replaced by a new one, 

 either in the same or in the following season. This has been experi- 

 mentally demonstrated in the case of several of the species. Three 

 "crops" of shoots of Asclepias mexicana have been harvested in one 



