AGRICULTURAL POSSIBILITIES OF ASCLEPIAS AND APOCYNUM. 61 



Before a fair idea can be formed as to the yields to be obtained, it 

 will be necessary to carry out some experiments in growing the plants 

 under field conditions. In the meantime, a study of the wild plants 

 and of the tables of analyses may give some hint as to what may be 

 expected. It is known, for example, that under favorable conditions 

 individual plants of the desert milkweed (Asclepias subulata) weigh 

 15 to 80 pounds. Perhaps 30 pounds, green weight, is a fair average 

 to expect when tillage and other methods to conserve moisture are 

 practiced. Such plants have a spread of 3 to 6 feet and require perhaps 

 3 years to mature, after which annual crops presumably could be har- 

 vested. It is even possible that 2 or more crops could be gathered in 

 one year, as is practiced with alfalfa, but the effect of frequent cropping 

 on the vigor of the plant is not known. Irrigation, even in slight 

 amount, would certainly increase the tonnage as well as permit closer 

 planting and more frequent cropping. It might also modify, either 

 one way or the other, the percentage of rubber produced. 



In carrying out estimates an allowance should be made for about 

 60 per cent of water in the green stems. But the determining factor 

 is the percentage of rubber present, and no definite prediction can be 

 made as to what this will be. The average content of 9 unimproved 

 plants growing at Sentinel, Arizona, exclusive of the woody base, 

 was 3.1 per cent. With this and the above figures as to size and other 

 factors as a basis, it would seem that between 300 and 500 pounds of 

 rubber per acre might be expected from each crop in case a uniform 

 stand of fair-sized plants can be secured. If it is assumed that the 

 highest percentage found in an individual plant could be uniformly 

 maintained under methods of seed selection, then these figures could 

 be at least doubled. Notable success in the improvement of other 

 agricultural crops through breeding combined with selection justifies 

 the hope that the average of 3.1 per cent can be increased to several 

 times this amount, with a corresponding increase in the total yield per 

 acre. Such improvement would be necessary as a preliminary to the 

 profitable cultivation of the desert milkweed for its rubber and fiber. 



In actual practice the yield would be considerably modified through 

 the inclusion of an undeterminable proportion of resins and other sub- 

 stances. The amount of such matter varies from about 6 per cent in 

 the best Para to about 35 per cent in guayule, and even higher in 

 some inferior Africas. 



To the income of the rubber is to be added the value of the pulp 

 for paper manufacture. As already demonstrated (p. 58) the air- 

 dry stems yield about 43 per cent of bone-dry fiber, or 28.5 per cent of 

 bleached paper. No figures have been obtained as to the value of the 

 pulp, but it would be higher than that made from hemp and similar 

 plants, since the separation of the fibers incident to rubber extraction 

 would be such as to render unnecessary certain preliminary processes. 



