Reproduction. 



63 



Normal retonos usually begin their growth with the oncoming of 

 rain, especially in spring and early summer. In this regard they act 

 merely as expressions of growth and have no special peculiarities. Start- 

 ing as they do from the shallow-lying roots, they make an etiolated growth 

 of a few centimeters before emerging from the soil. Their rate of growth 

 depends upon the size of the root from which they spring and the num- 

 bers arising at one point. If the root is slender growth is relatively slow, 

 and subsequently depends on the rate of secondary growth of its distal 

 portion; if large, the retoho grows rapidly and may in a month or two 

 attain a height of 10 or 1 5 cm., a rate scarcely to be met with in the case of 

 seedlings. A notion of the rate of growth may be had from the follow- 

 ing table of measurements, based upon the specimens in plate 9, fig. B, 

 the numbers referring to those similarly numbered in the figure. 



1 Dry weight i lb. 5 oz. 



2 Dry weight. 



3 Induced by cutting away the plant, January 1908. 



4 Grew in season of 1908. 



It is at once apparent that, as compared with the rate of growth of 

 seedlings, that of rotohos is much more rapid. It takes at least 15 years 

 to produce a plant of 2 pounds weight from the seed. Plant No. 1 , in the 

 above table, made its weight in certainly not more than 9 years, possibly 

 in 8. This is brought about by (1) the more numerous stems arising from 

 the base and (2) the more rapid elongation of the stems, due to the ad- 

 vantage had in the already established root-system. Table 20 affords 

 comparative data as between seedlings and retonos. Incidental advan- 

 tages accruing from this purely vegetative method of reproduction are (1) 

 relative certainty of success because of the previous establishment of the 

 parent plant, with relative independence of an initial good season in order 

 to start, and (2) the rapidity with which the plants arrive at a condition 

 to flower abundantly ; e.g., plant No. 1 1 , a few months old, produced fully 

 100 seeds. These, in a desert especially, are no mean advantages. Thus, 

 they would enable a single guayule plant to compete with such a plant 

 as the lechuguilla, assuming that it had so fully occupied the ground that 



