Reproduction. 89 



(fig. 14). Fig. 13, on the other hand, indicates a greater increase, to 

 nearly three times the weight at 30 cm. The average weight of a plant 

 30 cm. in height is, according to fig. 14, about 7 ounces, but as the plants 

 considered in this curve are normally developed or indeed considerably 

 above the average, the average weight of a 30 cm. plant is probably 

 nearer to that indicated in fig. 13, viz, 5 ounces. The mere ratio of 

 change in weight is not peculiar to these dimensions alone. What appears 

 from the data is that the weight of plants up to the height of 30 cm. is 

 not great enough for economical harvesting. The increase in size and 

 weight, however, is as great in the subsequent five years as in the previous 

 ten, so that taking the crop at the end of ten years would give results 

 only half as great as the returns of a fifteen-year rotation period. 



This conclusion is shown graphically by fig. 15, lower diagram, which 

 indicates that the weight of a plant advances from about 6 ounces at 

 ten years of age to 1 5 ounces at fifteen years. A considerable minus error 

 in the estimation of ages might be allowed, and yet the increase indicated 

 in the preceding paragraph would still be shown. It is fair to state, how- 

 ever, that there is little chance for such error, as I have taken the pre- 

 caution of being conservative when there was doubt. 



Fig. 15, upper diagram, indicates that the estimate of rate of growth 

 used throughout, viz, 3 cm. per year, is very nearly correct. "We may 

 therefore conclude: 



(1) That the average rate of growth of guayule per annum is about 

 3 cm. 



(2) That the amount of increase in weight between the tenth and 

 fifteenth years of its age is at least as great as that occurring during the 

 first ten years; and that this further justifies, from an economic point 

 of view, a fifteen rather than a ten year rotation period, aside from con- 

 siderations which might be drawn from loss by death (p. 71), could we 

 ascertain this accurately enough. 



