The Water-balance of Desert Plants. 
BY 
D. T. MACDOUGAL, Ph.D., LL.D. 
Director , Department of Botanical Research, Carnegie Institution of Washington. 
With Plates VI-X. 
T HE conducting tissues of seed-plants are closely connected with mor- 
phologically distensible tracts of medullary and cortical tissue which 
have an appreciable capacity for the retention of water in plants of even the 
strictest habit and stature. As the ascending current passes from the 
absorbent elements to the transpiratory surfaces, some of it may go into 
such masses of cells constituting reservoirs, in the roots, stems, or leaves. 
This accumulated supply may be drawn out to the transpiring cells when 
the pressure of the solution in the cell-sap is overcome. 
All plants with massive stems may thus carry a large balance of water, 
and this stored solution may play a very important part in the life of the 
individual. The relatively largest balances are carried by some of the 
species characteristic of the arid regions of the south-western and southern 
parts of North America, some parts of South America, and the southern 
part of Africa, while Northern Africa, Asia, Australia, and arid regions in 
high latitudes everywhere have but few plants with a large water-balance. 
The author began some work on the water-balance of desert plants in 
1908, and the earlier results obtained have already been published. 1 A few 
of the plants survived the original tests, and these with other living speci- 
mens have yielded evidence upon phases of the subject not discussed in full 
in the earlier paper. 
The Tucson region, in which the observations were made, has a winter 
rainy season and a wet midsummer, with a hot dry fore-summer and arid 
after-summer. The total average annual precipitation is about 12 inches.- 
The extremest arid effects are seen in June and early July, when the 
humidity falls as low as 6 per cent, with midday temperatures of no 0 and 
112 0 F. Some of the material was observed in the open, and other specimens 
were kept in a shaded laboratory room in which the extremes of temperature 
were not reached by ten or fifteen degrees. 
1 MacDougal, D. T., and Spalding, E. S. : The Water-balance of Succulents. Publication 
No. 141, Carnegie Institution of Washington, 1910. 
[Annals of Botany, Vol. XXVI. No. Cl. January, 1912.] 
