The Environment . 19 



It is seen, therefore, that at the depth mentioned the lower critical 

 growth temperatures in the soil are probably never reached, and it is 

 to be inferred that the dormant condition of the vegetation is deter- 

 mined by other factors, namely, soil-moisture and air-temperatures, and 

 of these the factor of moisture is probably the more effective. 



The temperatures affecting germination, however, are those of the 

 surface of the soil or at a very slight depth. Fig. 2 presents the curves of 

 air and soil temperatures for November 6, 1907, at a time when difficulty, 

 ultimately shown to be due to other causes than temperature, was ex- 

 perienced in germinating seeds in boxes. The soil-readings are for a depth 

 of 2 cm., and the soil was wet, but was exposed to full insolation. 



The temperatures from about 9 a.m. till 10 p.m. can not be said to 

 be unfavorable, though their effect upon the rate of germination and 

 subsequent growth would be offset by the succeeding hours of cool soil. 1 

 The cooler period is more marked during the succeeding months till 

 March or April (fig. 1). Inasmuch, however, as the night temperatures 

 are scarcely ever favorable for germination (assuming 40 Fahr. as the 

 lower limit) before June or after October, and even during this period 

 not especially so, we may conclude that the existing temperature condi- 

 tions at Cedros are of subsidiary importance in determining the time of 

 the year when germination occurs. This conclusion is supported by the 

 success attending germination tests made in January (Kirkwood, 1910), 

 when the temperatures ranged from 32 to 64 Fahr. At these tempera- 

 tures, germination did not begin so soon as when, later on, they were 

 somewhat higher. It therefore may be concluded that, aside from a cer- 

 tain rhythm which may be detected, winter dormancy both in the mature 

 plant and in the seed is due, in the area we are considering, rather to lack 

 of soil-moisture than to unfavorable soil-temperatures. This conclusion 

 can not, however, be applied throughout the whole of the guayule region, 

 since the winter temperatures in Texas are much more unfavorable. 



SOIL-MOISTURE. 



The residual soil-moisture during sustained periods of drought may 

 be reduced to a point below the minimum necessary to sustain life. This 

 is the chief cause of the local dying off of guayule during such periods. 

 Generally, however, the amount of soil-moisture, though insufficient to 

 stimulate growth even if other conditions are favorable, is more than 

 enough to sustain life, and indeed may be enough for growth when the 

 equilibrium between the plant and the environment is destroyed. The 

 results of certain experiments detailed beyond show this to be true. 

 Plants at station 2, quadrat 2, were pollarded in November 1907, about 

 5 to 8 cm. above the surface of the soil, and these had made a marked 

 growth by February 18, 1908, although the surrounding plants showed 

 no growth at all, and indeed did not until much later on. While there 

 had been a very small amount of rain, it was quite insufficient to account 

 for the growth, even in the pollarded plants, during the period between 

 the dates above mentioned. We may therefore conclude that usually 



1 Abbe, C, 1905, p. 36. 



