20 Guayule. 



during dormant periods the soil-moisture is considerably above the neces- 

 sary minimum, 1 but insufficient to stimulate to growth, although, on 

 account of lack of facilities, a quantitative statement can not be made. 

 This is to be regretted, because the peculiar distribution of the guayule 

 in the foot-slope, while Parthenium incanwm extends beyond its limits 

 into the playa, 2 is probably connected either with the superior water- 

 holding capacity of the soil of the foot-slope or with its greater air- 

 content, aside from the differences observable in the topography of the 

 root-systems of these plants. The naked statement that the guayule 

 is confined to slopes which are well drained 3 conveys little of explanation. 



RELATION OF RAINFALL AND TEMPERATURE TO GROWTH. 



Whatever is said here about the behavior of the guayule in regard 

 to growth-rhythm must be understood to apply to the region of North 

 Zacatecas, where the data which appear beyond in detail were obtained. 

 It is believed, however, that the generalizations are approximately true 

 for the whole area of distribution. 4 



The grand period of growth falls in the warm season, when super- 

 ficial soil-water is normally most abundant and when the night as well 

 as the day temperatures are most effective. If the rainfall is subnormal, 

 the drought so caused at this time results in very slow growth, made 

 possible only by the meager amount of water that reaches the plant 

 from the subsoil, derived in part from the earlier and usually small rainfall 

 of the previous winter, together with the more immediately available 

 supply from insufficient rains. This is only another way of saying that, 

 in the region above described, water, as compared with the otherwise 

 usually favorable conditions, is the prime condition for growth, and we 

 may best see what the habits of the plant are by observing what growth 

 takes place in relation to the rainfall. The extreme possibilities would 

 be expected to be shown by plants under irrigation during every season. 

 The observed growth in such plants, even in the presence of abundance 

 of soil-moisture during November, December, and January, is exceedingly 

 small in amount. Had the soil-moisture been reduced, say in Septem- 

 ber, so as to bring on a period of dormancy in the plant during October 

 and November, it may well be believed that a much more marked growth 

 might have occurred during the period following, when in point of fact 

 little growth actually occurred. This behavior would be in accord with 

 our general knowledge of growth-rhythm. 



Although I have made no observation of positive value in this 

 regard, it is said by supposedly competent observers that the guayule 

 in the field may be expected to flower at any time, and that it has been 

 seen to do so in every month of the year. Flowering, however, usually 

 involves some foliage-stem growth as well; and so the evidence favors, 

 or at any rate does not contradict, the view that growth may ensue at 

 any time of the year. Because of the unfavorable night air-tempera- 



1 Cf. Livingston, iqo6. 3 Escobar, igio. 



2 Tolman, see Spalding, iqoq. 4 Cf . Bray, 1906. 



