14 Guayule. 



range of the plant in altitude extends from the lower limit mentioned 

 to about 7,000 feet, or somewhat higher. As observed by Endlich (1905), 

 however, the most important acreage is not usually to be found much 

 above 6,000 or 6,500 feet. 



CLIMATE. 



The climatic conditions under which the guayule lives have not 

 only scientific interest, but very important practical bearings as well. 

 This will be understood upon the reflection that many proposed opera- 

 tions relative to the culture of the plant involve the use of water, and 

 whatever the theoretical possibilities may be, success on a large scale 

 must be conditioned very closely by the nature of the desert areas to 

 be utilized. The details in question will be considered in Chapters VIII 

 and IX. For these reasons a somewhat detailed account of the actual 

 climatic conditions observed at Cedros, in North Zacatecas, will be given. 



RAINFALL. 

 Fortunately, perhaps, for our purposes, the year (1907-08) during 

 which observations were begun was unusually dry, and afforded, we 

 believe, about the most rigorous conditions which the vegetation is 

 subjected to without marked unfavorable results. It is to be regretted 

 that data for the whole of this year can not be reported, since observa- 

 tions could not be commenced before the month of August. Relying 

 upon estimates and upon general, verbal reports, and judging by analogy 

 with the region about the city of Zacatecas, where the precipitation for 



1907 was about half (320 mm.) of the mean for 29 years (596 mm.), 1 it 

 seems reasonable to believe that the total rainfall for 1907 was not 

 greater than 175 mm. (7 inches), of which 138 mm. were recorded in- 

 strumentally as falling during the last four months. The growing season, 

 as would be indicated by the scant amount of rain which fell earlier in 

 the year, was a practical failure as regards crops in general, and the indi- 

 cations of growth in the guayule, which at this moment concern us most, 

 were consonant with the precipitation, which was at best very scanty. 



As will be seen upon examination of table 1 and fig. 3, the rainfall for 



1908 was somewhat over 10 inches, which appears to be about normal, 

 while the effective rains fall in the summer months. In 1 908 it was suf- 

 ficient to produce a prolonged period of relatively high atmospheric 

 humidity, while the replenishment of the store of water in the soil was 

 marked enough to produce very pronounced mesophytic conditions. In 

 the low-lying flats, especially where the more abundant collections of water 

 were formed, annual plants of weedy appearance grew densely breast- 

 high, and seedlings of the mariola scattered among them grew with great 

 rapidity to a height of 40 to 50 cm. in one season. On the low ridges and 

 in the hills the available stratum of the soil was full of water, and the 

 guayule and mariola, together with many other shrubs and annuals, 

 were in full bloom and making rapid growth in June. Other features 

 of the distribution of rains are indicated in a general way in the diagram 

 and are of importance as related to the period of growth of the guayule, 

 to be referred to beyond. 



1 Boletin Mensual del Obs. Astron. -Meteor. Zacatecas, Jan. 21, 1908. 



