How "living rock" cacti pull down in the soil

A number of cactus species are called “living rocks,” chiefly because their shoots grow little to no higher than the soil around them. According to field observations, these cacti recede further into the soil during unfavorable periods, particularly prolonged drought. One question about such recession is whether it is due to shoot shrinking, root contraction, or both. Changes in position relative to the soil surface were investigated for a “living rock” species, Ariocarpus fissuratus, and root and shoot anatomy were investigated for this species and others, including the so-called agave cactus, Leuchtenbergia principis. Fine wires were inserted into the base of the shoots at the soil line, and plants were grown in a glasshouse for four months, half receiving biweekly watering and half being watered bimonthly. The height of the wires and the soil moisture content were measured weekly. Plants in the drier treatment receded into the soil by as much as 22% of initial plant height (paired sample t-test, P = 0.04), whereas plants in the wetter treatment did not recede (P = 0.29). External examination showed transverse wrinkling in the lower, non-chlorophyllous stem region (originally the hypocotyl) as well as in basal (proximal) root regions; such wrinkling was more pronounced in plants from the drier soil, and suggested that both shoot and roots were involved in plant recession. Longitudinal sections made through roots of A. fissuratus and L. principis showed sinuous compression of the xylem, indicating that roots were contractile in both species.