120 Bulletin 79 



tion underlying the distribution of the great number of our cacti, 

 however, has not been made, although in the light of Mr. Uphof 's 

 experiments minimum winter temperatures are a factor. In this 

 connection it will be interesting to observe that during the very- 

 cold winter of 1912-13 thousands of small giant cactus plants 

 growing near their greatest altitudinal limits were killed outright. 

 Many other species also suffered great damage. 



For this study in hardiness, plants of spineless cacti — spineless 

 platopuntias — were secured from various sources, and, together 

 with those already growing on the University grounds, were set in the 

 introduction garden at the University Farm, in the spring of 1914. 

 The writer is under obligation to Fraser Bros., Wellton, Arizona, 

 for a number of varieties of spineless cacti, including Opuntia ficus 

 indica, from Sicily and Malta, and Opuntia sp. Burbank Special. 

 The latter was thought to be a very hardy variety. Mr. B. R. 

 Russell, San Saba, Texas, kindly furnished plants of Opuntia Ellis- 

 tana, which he calls the San Saba spineless pear. This latter species 

 is entirely spineless and has shown itself hardier than any other 

 variety of spineless cactus growing in the introduction garden. It 

 is, however, slower growing than some other species. Other species 

 represented were Opuntia fusicaulis, a slender jointed spineless 

 pear, which the writer found in cultivation in Tucson gardens, and 

 Opuntia castillae. These were recently described by Dr. Griffiths 

 and are native to Mexico. Opuntia castillae is known to have been 

 growing in Tucson in the gardens of various vSpanish and Mexican 

 residents for as long as thirty years and is commonly known 

 as "nopal de castilla." Forms of it are entirely spineless, though 

 the younger joints have a few fine spicules which sooner or later 

 disappear. It is often seen in cultivation in parts oi south- 

 ern California, where it is occasionally used as a hedge plant. Under 

 the most ordinary conditions it grows to a height of 6 to 10 feet, 

 the trunks and older branches becoming quite stout with age. 

 It is not injured by our highest summer temperatures, even with 

 rather arid culture, though for good growth it should have a rea- 

 sonable amount of soil moisture. It is well adapted for growing in 

 southern Arizona, where the lowest winter temperatures do not 

 fall below— 12.2° or —11.1° C.(10° or 12° F.). In December,1901, it 

 was not injured with a tempeiature of —12.2° C. (10°F.), extending 

 over a few hours. In January, 1913, it was frozen back one-half, 

 or more, on the University grounds, and also in Tucson with a 

 temperature of — 14.4° C. (6° F.). This latter temperature extended 

 over a considerable portion of the night and was preceded by nearly 



