Plant Ecology of the Sonoran Desert Region - Design with the Desert: Conservation and Sustainable Development - page 154

FIGURE 8.2
Succulence as a plant adaptation to life in arid environments. The large columnar cactus is saguaro ( C. gigantea ).
FIGURE 8.3
Succulence of an Agave that is a member of the Liliaceae family.
aloes, elephant trees, and many euphorbias. Several other adaptations are essential for the
water storing habit to be effective.
8.4.2 Getting Water
Succulents must be able to absorb large quantities of water in short periods, and they must
do so under unfavorable conditions. Because roots take up water by passive diffusion, suc-
culents can absorb water only from soil that is wetter than their own moist interiors. Desert
soils seldom get this saturated and don't retain surplus moisture for long. Desert rains are
often light and brief, barely wetting the soil surface that may dry out after just a day or
two of summer heat. To cope with these conditions, nearly all succulents have extensive,
shallow root systems. A giant saguaro's root system is just beneath the soil surface and
radiates as far as the plant is tall. The roots of a two foot tall cholla in an extremely arid site
may be 30 ft (9 m) long. Most succulents in fact rarely have roots more than 4 in. (10 cm)
below the surface and the water-absorbing feeder roots are mostly within the upper half
inch (1.3 cm). Agaves are an exception in lacking extensive root systems; they rarely extend
much beyond the spread of the leaf rosette. Instead, the leaves of these plants channel rain
to the plants' bases.