16 



Guayule. 



encountered in North Zacatecas. 1 Attempts which may be made in the 

 future in the cultivation of the plant, e.g., in New Mexico, must be made 

 with regard to its resistance to cold, and it is to be regretted, therefore, 

 that a final datum on this point can not be given. 



FIG. i. Maximum and minimum day and night temperatures by months; maximum 

 summer and minimum winter soil-temperatures at 10 cm. depth. Cedros. 



It will be noted upon examining fig. i that growing temperatures, 

 though sometimes low, occur even during the winter months in the day- 

 time. At night, however, the air-temperatures are seen to be practically 

 non-effective between the middle of September and the beginning of 

 May. This condition, judging from air-temperatures alone, may be 

 regarded as resulting in a functional resting-period of at least three 

 months; that is, the amount of growth possible in the year would be 

 that occurring within nine months of time, aside from the considera- 

 tion of rainfall. The soil-temperatures are of course higher, and are, on 

 account of the high insolation, frequently favorable for the absorption 

 of water by the roots, which would, under favorable conditions of soil- 

 moisture, be important in respect to the water-content of the plant, 

 though it might not, except when water was abundant or under other- 

 wise exceptional conditions, stimulate growth. The conditions as re- 

 gards growth, then, may be stated thus: The winter, or resting-period, 

 is effective during the night-time chiefly during October and on to the 

 end of April. The day temperatures during this period may effect growth 

 when water is sufficient. 



1 We now have records showing that guayule can stand a temperature of 5 F. 

 at Marathon, Texas, and of 10 F. at Tucson, Arizona. 



