The Environment. 21 



tures of the cooler period, however, the total amount of growth will not 

 be great, to which result the less effective growing day temperatures 

 contribute. The evidence shows further that growth in January, e.g., 

 will ensue upon a period of rest coupled with an unusually favorable 

 rainfall, spread over time enough to produce a marked rise in the avail- 

 able soil-moisture as far down as the shallow roots. The times at which 

 this conjunction of conditions may occur is indicated, negatively at 

 least, when it is said that no growth in field plants was observed till 

 May in spite of the rain, as indicated in table i and the accompanying 

 diagram (fig. 3). 



Not only, indeed, did no growth occur, but the guayule plants in 

 the field in widely separated localities showed a marked need of water, 

 a condition still more evident in April 1909. On the nth of Novem- 

 ber, at Jaguey, 10 miles northeast from Cedros, the leaves were in a 

 very much shriveled condition. Leaf -fall began toward the middle of 

 December, the upper leaves, which are not cast off, being at this time 

 in a distinctly flaccid condition. At this time the irrigated plants showed 

 signs of leaf-fall, but for some time only the lowermost on the season's 

 growth of stem were involved, while in the field plants all the fully de- 

 veloped leaves fell away at the same time. 



Although, as above, seen it appears probable that growth may 

 take place under favorable moisture conditions even in the winter, 

 there is little evidence (Chapter III) that the amount is ever anything 

 but small. The internodes are short, and thus is produced a crowding 

 of the leaves, which by summer growth would be spread apart, and the 

 structural marks between the two grand periods of growth are less ob- 

 vious. As will be seen later, the dependence which may be placed in 

 these marks as indicating the age of the plant is not materially disturbed 

 by this circumstance. 



RELATIVE HUMIDITY. 



Unfortunately no instruments were available at Cedros for the 

 study of relative humidity, and it is especially regretted that an atmom- 

 eter after Livingston's design was not at hand. The only data obtainable, 

 aside from my general observations, are those issued from the Observa- 

 torio de la Bufa at Zacatecas. A curve of tentative value based on these 

 is presented in fig. 3, and, while this can be regarded as only approxi- 

 mate, it serves to indicate that the relative humidity is relatively high 

 at Cedros (though not as high as at Zacatecas) , and that there is a some- 

 what prolonged summer period of high humidity. The following re- 

 marks accord in general with these conclusions. 



Dew is frequent during the cooler months, and was sufficient to run 

 off the roof of the house occupied as a laboratory, the material being of 

 painted canvas. The dew-point is always approached closely at night 

 and usually passed in winter and during the rainy summer season. The 

 high relative humidities which occur at all times during the night, and 

 in certain situations during the day, at least during growing periods, 

 are reflected in the vegetation. Only when this factor is taken into 

 consideration can we explain the pronounced contrast seen between 



