192 Guayule. 



The determination of the time at which the maximum rubber con- 

 tent is reached is of economic importance, as the earlier gathering of shrub 

 involves a considerable economic loss, amounting approximately to the 

 quantity of rubber secreted in one year in the new parts. If consistent, 

 therefore, with other considerations, the gathering of shrub should not 

 occur during, or for some time after the close of, the growing season. It 

 will be understood that by new parts is meant the new tissues within the 

 already secondarily thickened roots and stems, as well as new accretions 

 in length. The time at which the maximum amount of rubber may be 

 expected differs with the length of the growing-season, which depends 

 upon the rainfall and the intensity of the drought following. It thus 

 happens that field conditions are sometimes such as to produce results in 

 field plants (seedlings, table 54) similar to those in irrigated plants. 



10. The rate at which rubber is secreted by irrigated plants, under 

 the conditions described for the Cedros experimental plants, is such that 

 at the close of the second season's growth (Sept. 1908), the amount in 

 the cells is sufficient to agglomerate into the large masses characteristic of 

 field plants. This condition was, however, approached after a succeeding 

 drought-period lasting till April 1909 (plate 43, figs, i and 2; table 53). 

 In plants grown at Caopas under irrigation, during the first season's 

 growth (1908) and with a restricted amount of water during the second 

 season (1909), the amount of rubber was evidently greater than in the 

 Cedros material * and was great enough by October to agglomerate (plate 

 43, figs. 3, 4), forming dense masses, but not as large as in field plants. 

 There is, however, a large enough rubber-content in such plants for 

 mechanical extraction, though it is probable that some adaptation of the 

 process would be necessary. Although the amount of rubber may be as 

 low as 3 per cent, it must not be forgotten that the rate of growth under 

 irrigation is enormously in excess of that under field conditions. 



1 1 . There appears to be no direct physiological relation between the 

 secretion of rubber and of resin. 



12. Rubber appears to have no physiological function in the guayule 

 plant. 



1 The slowness of secretion in well-watered plants offers an interesting analogy 

 to the behavior of the rubber-bearing latex plant Castilloa elastica. (Collins, and 

 Pittier ; see Cook, 1 903) . Olsson-Seffer has also pointed out that the secretion of rub- 

 ber in this plant is retarded by irrigation, and in consequence it must be deprived 

 of water for some time before it can be tapped to advantage. 



