VARIATION IN KUBBER-CONTENT OF ASCLEPIAS AND APOCYNUM. 47 



B. Miscellaneous Families. 



305. Argemone platyceras (Papaveraceae), Manitou, Colorado. 

 938. Aristolochia calif ornica (Aristolochiaceae), Newcastle, 



California. 

 375. Comandra umbellata (Santalaceae) , Colorado Springs, Colorado. 

 1117. Convolvulus occidentalis (Convolvtdaceae), La Jolla, Califor- 

 nia. 

 1111. Convolvulus soldanella (Convolvulacese) , La Jolla, California.'' 



362. Croton calif ornica (Euphorbiacese), Brentwood, California. 

 806. Croton corymbulosus (Euphorbiaceae), Las Cruces, New Mex- 

 ico. 



828. Croton corymbulosus (Euphorbiaceae), Sonora, Texas. 



847. Croton texensis (Euphorbiaceae), Sonora, Texas. 



363. Ereinocarpus setigerus (Euphrobiaceae), Antioch, California. 

 1109. Fouquieria splendens (Fouquieriaceae), Tucson, Arizona. 



390. Gaura parviflora (Onagraceae), Manitou, Colorado. 



907. Hypericum perforatum (Hypericaceae), Grant's Pass, Oregon. 



812. Madura pomifera (Moraceae), Manhattan, Kansas. 



825. Microrhamnus ericoides (Rhamnaceae), Western Texas. 



378. Oenothera serrulata (Onagraceae), near Colorado Springs, 



Colorado. 

 989. Olea europaea (Oleaceae), Tucson, Arizona (cult.). 

 811. Othake sphacelatum (Compositae), nearGuymon, Oklahoma. 

 863. Psoralea tenuiflora (Leguminosae), Colorado Springs, Colorado. 



379. Smilax herbacea (Smilacaceae), Eastern Colorado. 

 1130. Stillingia aimua (Euphorbiaceae), Glamis, California. 



803. Stillingia silvatica (Euphorbiaceae), Northern Texas. 



848. StiUingia torreyana (Euphorbiaceae), Northern Texas. 

 881. Tragia ramosa (Euphorbiaceae), Royal Gorge, Colorado. 



IX. VARIATION IN THE RUBBER-CONTENT OF ASCLEPIAS 



AND APOCYNUM. 



Distribution of rubber in the plant. — Since the rubber both in 

 Asclepias and in Apocynwn is carried by the latex, it is not surprising to 

 find that the highest percentages are obtained from those plant parts 

 in which the laticiferous tissue is well developed. As will be seen from 

 an examination of the tables of analyses, the foliage nearly always 

 carries a much higher percentage than any other part. In all but one 

 species the amount carried by the steins is so slight, usually less than 

 1 per cent of their dry weight, that its recovery in commercial rubber 

 manufacture is perhaps not to be considered. However, the occa- 

 sional appearance of a plant in which the stem carries a fair amoimt 

 of rubber gives rise to the suggestion that improved strains might be 

 developed in which steins as well as leaves could be utihzed. Such a 

 plant is one of Asclepias mexicana (No. 917) which yielded on analysis 



2 A second sample of Convobml-us aoldaneUa, from the sand-dimes at La Jolla (No. 1114, April 

 3 1920) yielded 1.5 per cent of benzene extract from the stems, indicating that rubber may 

 possibly be present in small amount. 



