66 Guayule. 



The percentages of removed plants represented by new shoots in 

 all the above experiments are as follows: 



Per cent. 



Experiment 115 (pulling) 6 



Experiment 1 1 1 (cutting) 6 



Experiment 114 (cutting) '23 



Per cent. 



Experiment no (cutting) 10 



Experiment 121 (cutting) 12 



Experiment 125 (cutting) 15 



From the above data the following conclusions may be drawn: 



Retonos are formed much more easily from the stock left after cut- 

 ting at or near the level of the ground. The probability that the plants 

 removed will be represented by new growths after cutting is much greater 

 when a portion of the stem at the top of the tap-root is left. This is due, 

 of course, to the presence of numerous dormant buds. 



The promptness with which retonos start after cutting away the 

 plants depends, in the absence of sufficient soil-moisture, upon the rain- 

 fall. It is worthy of note (i) that these retonos may start slowly before 

 the advent of rain, and (2) that the roots may die back at least 13 cm. 

 before starting. Root No. i, experiment no, had died back 25 cm. dur- 

 ing six and a half months, that is, at the rate of about 4 cm. per month, 

 and it finally failed to start. It was a very dry period, and this long 

 tenacity of life illustrates in a striking way the physiological resistance 

 of these roots in desert conditions. 



While this degree of hardiness would serve very effectively to pre- 

 serve the species under unfavorable circumstances, it is evident from our 

 figures that the number of new plants produced is not as great as is gen- 

 erally supposed. The best result obtained (exp. 114) indicates that under 

 the conditions surrounding this experiment scarcely more than 25 per 

 cent of the original stand may be expected. It is a matter for satisfac- 

 tion, however, that even under the most drastic treatment a field of 

 guayule may be expected to reestablish itself in the course of time, since 

 the new growths will in a short time be able to produce seed and these 

 will contribute to the repopulation of the area. 



In April 1909, two areas were visited from which the guayule had 

 been removed by pulling up the shrub. It appeared that only the larger 

 plants had been removed, and that both places still contained the natural 

 growth of smaller plants. The point of interest in this connection is that 

 in one of the areas, the Lomerio de Zorrillos, it was very easy to find 

 broken-off roots which had started to grow again, and retonos of various 

 sizes up to 8 cm. were found. In the other area, in the Sierra de Ramirez, 

 the ground was very hard and the peons found difficulty in pulling the 

 plants out. Instead, they had twisted them off just above the surface, 

 and from the butts remaining, with very few exceptions, new shoots had 

 grown during the season of 1908, these measuring from 3 to 8 cm. in 

 height. This parallels the behavior of plants cut off at some distance 

 above the surface of the ground. 



Experiment 60. Station 2, quadrat i. 25 square meters. Nov. 5, 

 1907. 140 plants cut off at a height of 8 to 10 cm. above the sur- 

 face of ground. 



1 The total number was not determined in April 1909, but would doubtless 

 have indicated a larger percentage. 



