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 Carnegtea (18 inches high) has 
been discovered in Soldier Cafion 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 cafion. The 
plant is here well protected from the cold-air flow of the cafion 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. 
