The Cultivation of Guayule. 201 



which may then be taken. Between 30 and 40 cm. the maximum econo- 

 mic efficiency of growth obtains, and this lies between 10 and 15 years of 

 age. Fifteen years is therefore the rotation period, but as the growth effi- 

 ciency of a plant falls after this age has been reached, these plants must be 

 removed each fifth year. The advantage of this rule is to be expected not 

 only in the growth of the plants already there, but also in the great effi- 

 ciency of seeding. The question has been raised as to the possible increase 

 of efficiency of germination by partial or total clearing of the land, thus 

 removing the factor of competition. 



3. Removal of the vegetation other than guayule. It is too early to 

 make any definite statements as to the value, even with regard to the 

 well-being of the mature plant, of clearing operations on guayule lands. 

 The experiments which have been initiated involve an area of about 75 

 acres, which were well cleared of all vegetation excepting the guayule, the 

 "palms" (palma samandoca) which produce fiber, and the few cacti, of 

 large species, which occupy little area and do not constitute an aggressive 

 element in the vegetation. The clearing of the land has the effect of loos- 

 ening the more superficial layers of soil generally, and to some depth in 

 spots. On general grounds this ought (1) to remove competition with 

 other plants, which, as has been shown elsewhere, is not insignificant and 

 frequently constitutes a real menace to the guayule, necessitating partial 

 clearing, at least. This competition of course relates especially to the 

 water-content of the soil. Unless the removal of the co\*ering allows 

 greater washing than in any event occurs, it should render more water 

 available for the guayule and thus enhance growth. It must not be for- 

 gotten, however, that a much greater growth is correlated with reduced 

 activity in secretion of rubber, either directly or by reducing the volume 

 of the rubber-bearing tissues, as has been brought out in the discussion 

 of plants under irrigation and from areas of greater rainfall (Chapter V). 

 (2) It is important also to know what effect the removal of the vegetation 

 has upon the crop of seedlings. The evidence so far obtained appears to 

 favor the clearing of the land. I refer especially to the census of seedlings 

 made at Station 2 (page 70), in which are recorded numbers of seedlings 

 far in advance of those found elsewhere. As to the question of protec- 

 tion afforded young seedlings by the shade of other plants, of no small 

 importance in many cases, as has been repeatedly indicated by studies at 

 the Desert Botanical Laboratory, it may be concluded that the number 

 of seedlings which survive a six months' drought, as observed by myself 

 in April 1909, is sufficient to warrant the statement that not enough suc- 

 cumb to unfavorable conditions to neutralize the good effects, as seen in 

 the surviving numbers. It seems probable, therefore, that clearing the 

 land of other vegetation, saving the species above mentioned, is, on the 

 whole, beneficial to the guayule. 



As to the specific question of the response in growth, all that can be 

 said at this time is guesswork. The areas which were cleared, as it hap- 

 pened, were subject to severe droughts from the time of clearing in the 

 winter of 1907-08 till the summer of 1909. It is hoped that the abundant 

 rainfall of the season now drawing to a close will enable us to form some 

 conclusion on this point. 



