VARIATION IN RUBBER-CONTENT OF ASCLEPIAS AND APOCYNUM 53 



two or more "crops" may be obtained in a season, as in the case of the 

 well-known alfalfa or lucerne. Since this practice would greatly in- 

 crease the tonnage of herbage, its use in case the plants are brought under 

 cultivation is at once suggested. The cutting in itself, however, might 

 modify the percentage content of succeeding crops. While the data 

 at hand are not conclusive, they seem to indicate that the second crop 

 would carry as high or perhaps a higher percentage than the first, but 

 that the third crop might show a considerable decline, at least in plants 

 grown under unfavorable conditions. 



The evidence for these conclusions is as follows: A grain field in 

 Solano County, California, was infested with a scattered growth of 

 Asclepias mexicana. This had been mowed off with the grain in the 

 early summer of 1919, before a study of its rubber-content could be 

 made. By November 13 of the same year the new shoots had reached 

 maturity, and these were gathered from 80 different plants (sample No. 

 923). The dried and mixed leaves analyzed 4.1 per cent of rubber, 

 which is much higher than the average for the first growth of this 

 species and which is rarely exceeded even by single plants. Two 

 other collections of second-growth material, probably from one root, 

 were made near Mount Diablo, California, one on July 9, 1920 (No. 

 941), the other on October 12 of the same year (No. 1037). The 

 leaves of these analyzed 3.7 and 4.5 per cent respectively, the higher 

 percentage of the latter corresponding to its greater degree of maturity. 

 Again the percentage of the first crop is not known, but these returns 

 for the second crop are so high for A. mexicana that no considerable 

 falling-off could have taken place. 



The only evidence that the percentage may drop with the third crop 

 is supplied by studies of the above-mentioned plot near Mount Diablo, 

 and even here the result is evidently due to the poorly developed con- 

 dition of the third crop when harvested. The stems of this plot are 

 connected by horizontal roots, so that the percentage content may 

 be assumed as the same for all at any one time. On July 9, 1920, a 

 swath was cut through the middle of the second growth and the 

 leaves and stems together were found to carry 2.1 per cent of rubber 

 (No. 941; leaves 3.7 per cent, stems 1 per cent). By October 12 it 

 was found that the shoots (stems and leaves) not cut in July had 

 increased their rubber-content to 2.4 per cent (No. 1037; leaves 4.5 

 per cent, stems 0.8 per cent), whereas the third crop from the cut 

 swath yielded only 1.5 per cent (No. 1038). The shoots of this third 

 crop were so weak and slender that the leaves and stems were not 

 analyzed separately. It is possible that by giving the plants the 

 proper cultural conditions, and by so timing the harvests as not 

 seriously to weaken the roots, a third crop may be obtained which 

 would be as rich as either of the other two. The effect of cutting 

 upon the vigor of successive crops has been noted only under the 



