58 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
Another plant, the sahuaro or giant cactus (Cereus giganteus), has given 
quite as striking results, and the extent of its response has been carefully 
measured. By means of an extended series of exact measurements, 
continued at intervals through a period of nearly four years, Mrs. E. S. 
Spalding (1905) has shown that the sahuaro, by a bellows-like action 
of its ribs and furrows, possesses a means of very perfect adjustment to 
varying amounts of stored water, and that its action is promptly set up 
when even a slight rainfall, after a period of drought, provides the neces- 
sary increase of water-supply. Plate 23 shows expansion and contraction 
between two points on opposite sides of a furrow of the giant cactus from 
March 3, 1906, to May 26, 1907. The broken portion covers a period 
in which no observations were made. Each horizontal space represents 
one day, and the days on which measurements were made are indicated 
by the dates. Each vertical space represents ¢; inch, and the figures 
at the sides the distance between the points in question in inches and 
sixty-fourths. Thus, on March 3, 1906, the points were 2}; inches apart, 
and on March 8, 24? inches, and so on. Below is shown the rainfall for 
the same period, each vertical space representing one-tenth of an inch 
of precipitation. When the rainfall amounted to only a trace it was not 
recorded. The complete record of precipitation for the period corre- 
sponding to the time for which the curve was drawn is given in table 
6, page 95. 
The giant cactus, rising as it often does to a height of 50 feet more or 
less, in the form of a gigantic fluted column, which may be simple or 
branched, is, mechanically speaking, a huge reservoir of water, subjected 
to the stress of high winds, and so constructed that for a long period of 
years it not only maintains securely its erect position and steadily con- 
tinues its growth, but also promptly expands whenever the soil is wet 
by rain, even for a short distance. The construction of such a tank 
represents an engineering feat which probably has no parallel in any 
artificial structure in existence, and the case appears still more remark- 
able when it is considered that this whole system of storage in an adjustable 
tank is dependent for its highest efficiency on the peculiarities of its root- 
system. ‘These are discussed by Dr. W. A. Cannon in the following pages. 
