MacDougaL — T he Water -balance of Desert Plants . 87 
Echinocactus was seen to exhibit both root development and apical 
growth of the stem after one or two years of depletion of the water-balance, 
and showed some capacity also for flower formation. Individuals exposed to 
the full intensity of the Arizona sunlight might not survive for more than 
a year, although even the slightest amount of shade would greatly enhance 
the value of the enormous water-balance. Plants in an ordinary room were 
in good condition after three years of deprivation of water. 
Repeated observations show that the flattened Opuntias may exist for 
extended periods, perhaps two or three years, without a water-supply, and 
may carry out seed-formation during this period. New joints may be 
formed, but generally at the expense of the older ones, which are destroyed 
during the process. 
Ibervillea has a great balance of food material as well as water, which 
is accumulated in woody tubers with heavily indurated outer tissues. These 
plants have been seen to be capable of developing the thin shoots for many 
years with a small rate of depletion. One individual has been under 
continuous observation for ten consecutive seasons. The intensity of the 
illumination and temperature, of course, figure largely in the matter. 
The corms of Brodiaea respond to seasonal stimuli by the development 
of smaller new and turgid corms in prolonged periods of desiccation. The 
older corms are emptied and destroyed in the process, which may be 
repeated a few times. 
No notable morphogenic departures were seen in the phenomena of 
growth and reproduction as exhibited by Echipoc actus, Opuntia , and 
Ibervillea with a depleted water-balance, except that the new structures 
were generally of a minimum size. The stems of Dioscorea developed under 
such conditions, however, showed some notable departures towards a sclero- 
phyllous habit. (See pp. 83-5.) 
All of the plants examined showed a high rate of water loss immediately 
upon removal from the soil, due to evaporation from moist outer surfaces 
and from abraded tissues, regardless of the season. Next it was found 
that the curve of transpiration was lowest in the cool months of November, 
December, and January. At this time the daily rate of Echinocactus was 
between one in 40,000 of the total weight and one in 8,000 in a well 
lighted room, while individuals in the open showed a rate of one part in 
5,000 to one part in 2,500, and even one part in 300 in a single instance. 
The maximum rate during the hot, dry fore-summer with relative humi- 
dities between 6 and 20 per cent, and midday temperatures as high as 
90°F. indoors and H2°F. in the open, varied between one in 800 and one 
in 4,000 in a well lighted room, while it rose to one part in 300 for the 
entire period of February to May, and must have been double this rate 
in May. 
The minimum daily rate of loss in Carnegiea (adults) was about one part 
