Coi^d-Resistance in vSpineivEss Cacti 121 



24 hours of almost continuous freezing weather. This is said to be 

 the lowest temperature ever recorded for Tucson. At Phoenix, 

 Arizona, this species was killed back about one-fourth to one-fifth 

 during this same cold spell. 



The soil in the introduction garden is deep and alluvial in 

 character. Ic is a fine, sandy loam and has good drainage. Some 

 black alkali is present, though apparently this has not injured the 

 growth of these cactus plants in the least. The cuttings were 

 planted on low ridges and cultivated and irrigated between the 

 rows, all the plants being given the same care. The soil has been 

 kept in good condition during the growing season of the plants. 

 The various plants have made at best but a moderate growth, par- 

 ticularly when the favorable growing conditions are considered. 

 Opuntia castillae, Opuntia ficus indica, Opuntia fusicaulis, and 

 Opuntia sp. Burbank Special grew to heights of 14 to 24 inches 

 during the first year with 4 to 8 joints each. Plants of Opuntia Ellis- 

 iana made growths of 10 to 12 inches with 2 to 5 joints each. As 

 already noted, this is a slower growing species than the others, but 

 very hardy. The winter of 1914-15 was milder than usual and 

 little damage from frost was done to the matured growth of any but 

 the tenderest species, viz., Opuntia ficus indica, 0. fusicaulis, and 

 Opuntia sp. Burbank Special. The lowest temperatures at the Uni- 

 versity grounds for the winter season were — 4.4° C. (24° F.) on 

 December 9, and —5.6° C. (22' F.) on the nights of December 15, 

 1914, and January 8, 1915, respectively. These temperatures were 

 of short duration. Opuntia Ellisiana and 0. castillae were not 

 injured in the least, while Opuntia fusicaulis, 0. ficus indica, and 

 Opuntia sp. Burbank vSpecial were only slightly injured. The 

 greatest injury was done by the freeze of December 8, since up to 

 that time the weather had been mild, and some of the plants were 

 making considerable growth. Naturally, the tender immature 

 joints were easily killed. Altogether, the plants came through the 

 winter season in good shape and began a healthy growth in the 

 spring of 1915. 



It is interesting to observe that Opuntia castillae and 0. Ellisiana 

 ceased their growth by the middle of October, or the first of Novem- 

 ber at latest, even with favorable growing conditions. Opuntia ficus 

 indica, 0. fusicaulis and Opuntia sp. Burbank Special, on the other 

 hand, continued growth ordinarily until freezing weather began in 

 December. There may be some relation between hardiness in 

 Opuntia castillae and O Ellisiana and their better adaptability to 

 our climatic conditions. This, however, might account for hardiness 



