INTRODUCTION. 3 



the desert and sea-shore. The desired information could only be obtained 

 by observations extending over a number of years, a fact that accounts for 

 the dearth of evidence upon the general subject. 



PHYSIOGRAPHIC, CLIMATIC, AND FLORISTIC FEATURES OF THE 



SONORAN DESERT. 



The region in which are found the plants that were made the subject of 

 the observations detailed in this paper consists of a series of mountain 

 ranges with a general trend northwardly and southwardly, the intervals 

 between neighboring ridges having the aspect of being broad plains or 

 valleys, and being in reality troughs filled with material worn down from 

 the mountains and spread out in such a manner as to make a series of 

 layers, hundreds or even thousands of feet deep. The gentle slopes, or 

 bajadas, leading away from the mountains, are generally devoid of water, 

 and it is only in the lower parts of the valleys, along the streamways, that 

 water is to be found within such distance from the surface as to be avail- 

 able for even deeply-rooting plants. In such places the vegetation may 

 include forms characteristic of humid regions. The spinose and succu- 

 lent xerophytes with which this paper is especially concerned inhabit the 

 bajadas and the rocky slopes of the mountain. 



The total annual precipitation amounts to about 30 cm. at elevations 

 below 1,250 meters altitude, and more than one-half of this amount is 

 received in the violent torrential rains of July and August, each downpour 

 causing the channels of the steeply-graded streamways to run with a tor- 

 rent which quickly subsides as the rain ceases. The remainder of the 

 precipitation is more evenly distributed throughout December, January, 

 and February, with some in March and April and perhaps in November. 

 The general effect of the precipitation is to increase the soil-moisture of 

 the surface-layer of the bajadas to a depth of a meter or less, and the 

 greater number of native species occupy more than half this depth with 

 their roots. (Fig. 1.) 



The sahuaro {Carnegiea gigantea) forms a tall, columnar trunk with 

 thick branches, which may reach a height of 25 meters or more, and it de- 

 velops a short tap-root and many horizontal branches which lie from 20 to 

 50 cm. below the surface. The bisnaga (^Echinocactus) has a thickened 

 cylindrical trunk rarely attaining a length of more than a meter, with a 

 diameter half as great. Many roots issue from the base of the stem and 

 ramify in a superficial layer of the soil, not penetrating more than 12 

 to 18 cm. deep, thus lying above those of the sahuaro when the two are 

 found in contiguity. The irregularly branching stems of the prickly pears 

 ( Opuntia) consist of many flattened joints and a very short basal trunk 

 from which the roots issue to spread horizontally through the upper layers 

 of the soil. (Plate 1.) 



