18 RUBBER-CONTENT OF NORTH AMERICAN PLANTS. 



weighed from 12 to 16 pounds, and H. R. Reed has collected at Dome, 

 Arizona, plants with over 500 stems and weighing 25 to 30 pounds. 

 The largest plants thus far located were discovered by Dr. F. E. 

 Clements at a point about 15 miles east of Mesa, Arizona. One of 

 these measured 6 feet high, 7 feet broad, and had approximately 2,000 

 ultimate branches. It was estimated to weigh close to 100 pounds. 



The difference just mentioned in regard to habit raises the question 

 as to the possible presence of two distinct races or subspecies. There 

 is some evidence to indicate that this is the case. The woody type, 

 as one form may be called, grows mostly on the bench-lands and 

 stony slopes, sometimes also in dry streamways. It is found as 

 scattered individuals and withstands excessive drought, as shown, for 

 example, by its presence near the very dry and well-drained summit of 

 Picacho Peak, California, at an altitude of 1,945 feet. The plants of 

 this form are almost shrubby, the lower portion of the stems being 

 decidedly woody. The branches are comparatively few, commonly 

 about 30 or 40 at midway from the ground, and are not widely spread- 

 i ng. Individuals as much as 8 feet high have been noted on Picacho 

 Peak by Mr. Reed. Perhaps this form is to be identified as A. albi- 

 cans Watson (Proc. Am. Acad. Sci. 24: 59, 1889). 



The second form, which may be spoken of as the stooling type, has 

 been found only in dry streamways, or in depressions, or on plains 

 where apparently there is a little more moisture than at places where 

 the woody type occurs. Yet even in these more favored spots the 

 conditions are decidedly arid and the plants are fully exposed to desert 

 conditions. These statements apply only to plants of Arizona and 

 southern California, since the conditions under which they grow in 

 Mexico are not well understood. The plants of this type are woody 

 only at the very base, which branches or stools near the ground into 

 a large number of erect or ascending reedy stems. A specimen is 

 illustrated in plate 1. The number of stems varies from about 100 to 

 600 or more, and since they average fully as long as in the other type, 

 the total weight of the plant is usually much greater. Small plants 

 will weigh from 5 to 15 pounds; exceptionally large ones are estimated 

 to weigh 50 to 100 pounds. 



In comparing the two forms of Asclepias subulata as to their relative 

 value for agricultural purposes, it is first noted that the stooling type 

 will yield much the greater tonnage, that its growth is more rapid, 

 and that it could be more easily harvested by mowing machines. On 

 the other hand, the woody type is more drought-resistant and carries 

 a lower percentage of moisture. The moisture-content of two woody 

 plants, as determined by Mr. Reed, was 43 and 49 per cent, respectively, 

 while one plant of the stooling type contained 60 per cent. The 

 samples were taken 8 days apart and from different localities, which 

 may account for the difference, but this ratio is about what one 



