Reproduction. 85 



The smaller amount of growth in 1909 was due to the absence of 

 irrigation, as elsewhere explained. It will be noted that the medium- 

 sized plant responded best, which in general comports with our observa- 

 tions of the rate of growth of field plants. 



GENERAL CONCLUSIONS. 



The maximum rate of growth of guayule under irrigation is in the 

 neighborhood of 25 cm. per year stem-length. The amount of growth 

 between the field average and the maximum average for irrigated plants 

 may be closely regulated by irrigation, to which the plant readily responds. 



FIELD PLANTS. 



When it is borne in mind that the total height of a plant is, except 

 in young seedlings of strict habit, always less than the sum of its longer 

 annual growths, because of (a) partial dying back and (6) the branching 

 habit ; and when also it is remembered that numerous plants suffer from 

 untoward conditions, either by the depredations of parasites or from 

 poor soil-conditions, it is not far from the truth to say that the average 

 annual rate of increase in height is 3 cm. A plant 30 cm. in height would 

 therefore be 10 years old. Plant 3, above described (p. 83), which has 

 undoubtedly a higher rate than 30 mm. per year, is, as certainly as may 

 be estimated, 10 years old. As has been said previously, however, the 

 important desideratum is to determine the period of life during which 

 the increase in weight is most rapid, aside, of course, from the very young 

 seedling stages, when the ratio of increase may be rapid, but the total 

 weight very little. For the purpose of arriving at this information, I 

 have assumed the rate 3 cm. per annum as a constant factor. A large 

 number of plants have been weighed and measured, and the data thus 

 derived have been correlated so as to obtain curves of increase in weight 

 according to size (fig. 13). For the data the reader is referred to tables 

 4 to 13. 



The curves have not been constructed for plants over 40 cm. in height 

 for two reasons : the number of plants beyond this size is very much smaller, 

 and, again, their age is too great to admit them to a practical considera- 

 tion of rotation periods. Observations from which, in part, the tables of 

 data used in the construction of the curves have been derived, all go to 

 show that the first pronounced gain in weight is entered upon after the 

 plant has reached a height of 30 cm. The average weight of plants of this 

 height is somewhat over 5 ounces, ranging chiefly between 2.5 and 8.5 

 ounces. The average weight of plants 40 cm. tall is, on the other hand, 

 15 ounces or more. That is, the average weight is trebled in making the 

 10 cm. advance in height beyond 30 cm. This is shown in the positions 

 of the curves, which, however, present more irregularities than one would 

 wish, in spite of the fact that they are based on measurements of several 

 hundred plants. The greatest fluctuations in the curves are caused by the 

 introduction of exceptional individuals, for where larger numbers are used 

 the curves are more uniform. The exceptional individuals may be either 

 "spindling" or unusually well-developed in point of ramification. 



