72 MacDougal . — The Water-balance of Desert Plants . 
Echinocactus Wislizeni. 
This species is native to the Tucson region, being found on rocky slopes 
between 500 and 9C0 metres elevation. The globose or cylindrical stems 
attain a length of 1*5 metres and a diameter of half or even two-thirds the 
length in some instances. The root-system is horizontally disposed. 1 The 
stems may have a weight of 80 to too kg., and the long axes are generally 
parallel to the axis of maximum illumination, which accounts for a slight 
leaning to the southward in a perfectly open country. The southern side 
has the folds more closely drawn together, while the cortex of the shaded 
side will be more heavily developed with the external ridges of the stem 
widely separated. The extra weight on the shaded sectors of the plant 
counterbalances the tendency to fall towards the southward. (See PI. VI.) 
About twenty individuals of various ages were examined during 
a period of four years. It will be necessary to summarize briefly some of 
the data already published in order to interpret the newer facts presented. 
Echinocactus No. 1. A large plant weighing 42*743 kg. was taken up 
March 4, 1908, and placed on a suitable support on a platform in a shaded 
room. The rate of loss at the beginning was 17 g. daily or one part in 
2,500, which rose to 29 g. daily in the arid fore-summer, dropped to 14 g. 
during the summer rains, rose to 18 g. in the dry after-summer in Sep- 
tember, and then fell irregularly to 1 g. daily in December. This rate 
of one part in 40.000 is the lowest recorded for any plant that came 
under observation. The highest daily rate, one part in 1,100, was observed 
in May, 1908. A year later, when the plant had lost about 4 kg. or 10 per 
cent, of its original balance out of a total weight of nearly 43 kg., the 
daily rate was but one in 2,500 of the total weight. With reference to the 
amount of the water-balance, the daily rate was one in 1,300 in May, 1908, 
and one in 2,000 in May and June, 1909, at which time the weight was 
about 38 kg. 
Echinocactus No. 2 was taken up November 5, 1908, and placed in 
the room with No. 1, weighing 5*136 kg. The midwinter rate of loss was 
as great as that of No. 1, but it soon fell to o*6 g. daily. This was equivalent 
to one part in 8,056. The plant was placed in an equable temperature 
room in mid-February in which the thermometer showed a rise from 56° F. 
to 71 0 F. in midsummer, and the humidity ranged from 80 to 90 per cent. The 
rate under such circumstances rose to 0-7 g., seemingly independent of 
humidity, with a direct relation to the temperature at first, but the rate 
continued to increase despite the falling temperature until it reached 1*3 g. 
daily in October. A decrease now ensued parallel to the falling tempera- 
ture which carried the rate down to 0*27 g. daily, or one part in 18,000. 
1 See Cannon, W. A.: The Root -habits of Desert Plants. Publication No. 13T, Carnegie 
Institution of Washington, 1911. 
