328 AG Tansley. 
On this afternoon the highest air-temperature encountered on 
the tour was registered. In the Middle West we had more than 
once had temperatures of over 100°F., but at Mecca a thermometer 
showed 115°F. in the shade. On the shore of the Salton Sea, where 
the relative humidity is high for such temperatures, owing to the 
proximity of the great body of water, the heat was decidedly trying. 
In the date-orchard the screened thermometer registered 108°F. at 
5.30 p.m. 
Tucson and the Arizona Desert. 
During the night of September 19th we continued our railway 
journey along the edge of Salton Sea and crossed the Colorado 
River into Arizona only seven miles from the Mexican frontier. In 
the morning we were travelling through the Arizona desert and 
approaching Tucson, and here we caught our first glimpse of that 
imposing plant Ccirnegia gigantea (Cereus gigauteus), which stands 
up here and there as a column 20 or 30 feet high in the midst of the 
desert scrub. 
The whole of September 20th was spent at the famous Desert 
Laboratory two or three miles outside the town of Tucson, on the 
lower slopes of Tumamoc Hill. Here Dr. MacDougal and the staff 
of the Laboratory had spared no pains to make our visit as pleasant 
and profitable as might be during the short time available. The 
different members of the staff gave short informal accounts of the 
researches being conducted at the Laboratory, many of them of 
great interest and importance. Dr. Forrest Shreve gave us an 
instructive little talk on the climate of Tucson and its relation to the 
vegetation. Dr. Livingston gave a demonstration of his atmometer 
cups, now widely used throughout the United States. 
This is not the place to give any account of the work carried on 
at the Desert Laboratory, which is fully dealt with in the annual 
reports issued by the Carnegie Institution and in the other 
publications emanating from this important centre of research. It 
is sufficient to say that Dr. MacDougal and his fellow-workers are 
continuously making determined attempts to penetrate to the more 
fundamental questions of desert ecology, and with a large measure 
of success. 
The vegetation of the neighbourhood of Tucson is fairly familiar 
to ecologists from the papers of Dr. MacDougal, Dr. Spalding and 
other workers at the Desert Laboratory, and no good purpose 
would be served by attempting a necessarily superficial description 
of it here. The country may fairly be called “ semi-desert,” since it 
has a distinct summer rainfall period in addition to the winter period 
