96 VEGETATION OF A DESERT MOUNTAIN RANGE. 



affect the exact altitude at which the maximum number of freezing 

 hours may operate in the limitation of a given species; and the exact 

 topographic character of the location of an individual plant may also 

 affect the operation of this factor. 



The experimental work of the writer has shown that a duration of 

 more than 18 hours of freezing temperature is fatal to Carnegiea and 

 that Opuntia versicolor and Echinocereus polyacanthos are capable of 

 withstanding durations of 66 hours. The limitation of Carnegiea is 

 apparently due to the operation of this factor. Its occurrence becomes 

 confined to south slopes at 4,000 feet and it becomes less and less 

 abundant from that elevation up to 4,500 feet. One of the highest 

 individuals at the latter elevation is protected by a rock on its north 

 side, above the summit of which the cactus now projects for 8 inches. 

 This projecting top was badly frosted on its north side in the severe 

 winter of 1912-13, while the north side of the plant below the summit 

 of the rock was uninjured. A small Carnegiea (18 inches high) has 

 been discovered in Soldier Canon at 5,100 feet. It grows on the south 

 side of a low rock, and its location is on the steep south slope which 

 terminates a long ridge between two main branches of the canon. The 

 plant is here well protected from the cold-air flow of the canon and is 

 subjected to the full insolation of the short winter days. It showed 

 some slight effects from the exceptionally cold winter just referred to, 

 but succeeded in recovering from them. In the early arid fore-summer 

 of 1911 the writer transplanted a young Carnegiea 3 inches high from 

 the base of the mountain to the vicinity of the 6,000-foot station on 

 Manzanita Ridge. The cactus was placed on the southwest side of a 

 rock, with a large plant of Arctostaphylos northeast of it, and occupied 

 a location near the summit of the ridge. The plant was watered 

 several times in order to help it to become established, but was not 

 assisted after the commencement of the summer rains. It success- 

 fully passed the winter of 1911-12; it made gains in turgidity in the 

 summer of 1912, but no measurable growth; in the spring of 1913, 

 after the winter in which the minimum temperature at that locality 

 was 6, the plant was found to be dead. Although the rainfall 

 at Manzanita Ridge in the summer of 1912 was 8.68 inches as com- 

 pared with 5.61 inches at the location from which the cactus was 

 taken (near the 3,000-foot station), it was not able to seize the 

 advantage. This fact itself involves the factor of summer temperature, 

 which doubtless determines the rate of growth of the roots and their 

 power for the intake of water. 



The evidence which shows Carnegiea to be limited in its upward 

 distribution by the greatest number of freezing hours is probably 

 applicable to a large number of desert plants, non-succulent as well 

 as succulent forms, which find their limitation at about the same 

 elevation. 



