CONTENTS. 



I. Purpose of the investigation 5 



II. Earlier investigations 6 



III. Acknowledgments ' . . 12 



IV. Methods employed 12 



Collection and preparation of samples 12 



Chemical analysis 13 



V. The most important species 15 



The genus Asclepias, or true milkweeds 15 



Asclepias subulata (desert milkweed) 17 



Asclepias sullivanti 22 



Asclepias syriaca (common nulkweed) 24 



Asclepias califomica 26 



Asclepias latifolia (broad-leaf milkweed) 29 



Asclepias mexicana 31 



Asclepias galioides (whorled milkweed) 33 



Asclepias brachystephana 34 



Asclepias speciosa (showy milkweed) 35 



Apocynum cannabinum (Indian hemp) and A. androsiemifolium (spread- 

 ing dogbane) 37 



VI. Miscellaneous species 39 



Acerates auriculata 39 



Asclepias eriocarpa 39 



Asclepiodora decumbens 40 



Crjrptostegia grandiflora 41 



Jatropha cardiophylla 41 



Hymenoxys floribunda utilis 41 



VII. Species with very low rubber-content 42 



VIII. Species examined with negative results 46 



IX. Variation in the rubber-content of Asclepias and Apocynum 47 



Distribution of rubber in the plant 47 



Variation due to heredity and to environment 48 



Seasonal variation 50 



Variation in successive crops 52 



X. Properties of the rubbers .%. 64 



XI. By-products from Asclepias and Apocynum 56 



XII. Agricultural possibilities of Asclepias and Apocynum 60 



XIII. Summary and conclusions 63 



XrV. Literature cited 65 



