274 



THE PLANT WORLD. 



if-.--. 



e^ 



on either side. P'or this reason it has been referred to not in- 

 advertently as " a desert in an oasis." 



The surface, sloping noticeably to the west, is ridged, thereby 

 dividing- the ground into many small areas a few to twenty feet 



in extent. It will be inter- 

 j esting to note here that the 



soil for this purpose was nec- 

 essarily hauled in. the soil 

 cover in the garden averag- 

 ing but twelve inches in depth, 

 below which there is a more 

 or less impervious calcareous 

 deposit several feet deep, 

 known locally as " caliche." 

 By virtue of the small ridges 

 already noted, the excess of 

 water from the torrential sum- 

 ^ ' mer showers and the less fre- 

 ' '' quent winter rains is pre- 

 ^ vented f r om running off. 

 Thus all the annual rainfall, 

 which averages about twelve 

 inches for Tucson, and which is usually sufficient for the nor- 

 mal growth of these plants, becomes available for them. The 

 garden is, however, irrigated occasionally, i. c, during periods 

 of lessened precipitation or of prolonged drought, the intention 

 being always to supplement rather than supplant the scant rain- 

 fall. As a matter of fact it has never been found needful to 

 water the garden oftener than once or twice a year, notwithstand- 

 ing that periods of six months will pass with a precipitation of 

 two inches or less. 



About six hundred numbers have been planted, representing 

 one hundred and fifty species from Arizona, California, Texas, 

 and a few from Mexico and South America. Most of these plants 

 from the two latter countries, and some from higher elevations 

 have died out, owing, perhaps, to unfavorable conditions. The 

 loss is apparent, however, only when a study of the garden list 

 is made, since new ones have been added from time to time and 



Fig. 45. Ocotillo. 



