398 Cannon: Transpiration of Fouquieria 



factors from day to day. For outdoor work, therefore, where the 

 conditions cannot be controlled beyond a certain limit, it is a great 

 advantage when the climatic factors vary in a uniform manner, or 

 when certain conditions are so pronounced that their influence is 

 unmistakable. The year 1904 was an especially fortunate one for 

 field work because, as will be shown below, uniform variations in 

 the temperature of the air, in humidity and in other climatic fea- 

 tures were maintained for days together, and the uniformity of the 

 environmental factors made the outdoor work similar in this regard 

 to that done in the laboratory where the conditions of experiments 

 are to a degree under control. 



Apparently the main cause of the uniformity of the climate in 

 1 904 was the small rainfall of the year. At the same time the 

 most disturbing feature of the uniformity was the sudden and in- 

 frequent rains. Since the rain is an all-important factor in the 

 biology of desert plants, it consequently happens that the history 

 of their transpiration reflects most clearly the influence of rain and 

 of drought, so that the relation of transpiration to these features 

 constitutes the key-note of the studies. For this reason it is best 

 before presenting an account of the experiments to give an outline 

 record of the climate for the year, in which emphasis is placed on 

 the humidity of the air and on the rainfall. 



Climate for 1904 



The normal rainfall at Tucson, the average for iS years, is 

 11.74 inches.* In 1904 the total precipitation was 7.79»t ^^ 

 about 75 per cent, normal. 



The distribution of the rains throughout the year, both the 



T "11 



usual distribution and that for 1904, is shown in figure i. It win 



observed 



were 



very Ught, but the summer rains, particularly those of August, 

 were slightly greater than usual. This distribution operated there- 

 fore to intensify the dryness of that portion of the year when the 

 plants are naturally least active, and at the same time to bring 

 about unusually favorable vegetative conditions at the season 

 the greatest activity, that is, in summer. 



of 



* Coville, F. v., & MacDougal, D. T. Desert Botanical Laboratory of the Car- 

 negie Institution, 26. I903. 



t From data provided by the University of Arizona. 



