How "Living Rock" cacti pull down in the soil

Abstract

Ariocarpus fissuratus , commonly called ?living rock? cactus, is known to keep its shoot relatively even with the substrate. The mechanism behind this trait is largely unknown; however, it is believed to be a response to environmental conditions. Root contraction, shoot contraction, shoot shrinkage or a combination of these factors could be responsible for keeping the shoot at soil level. Wires were inserted near the soil line in the A. fissuratus shoot to track its position relative to the substrate. Permanent ink was used to make marks 4-11 mm apart along the vertical axis of shoot to track shoot contraction, and a digital camera and an imaging program were utilized to track the projected surface area. Weekly measurements were taken for three months during the winter of 2006-2007 and over a 4-8 week period in summer 2007. Two groups (n = 11) received a varying watering regiment to investigate the role of water stress on root contraction; one group was watered weekly and the other once every three weeks. A repeated measures ANOVA showed no significant difference between groups for wire height (P = 0.445), shoot contraction (P = 0.780) or surface area (P = 0.374). However, over time, both groups showed a significant decrease in wire height (P = 0.042) and shoot contraction (P = 0.046), whereas surface area showed no significant change over time (P = 0.283). Anatomical investigations of vascular tissue indicate that A. fissuratus roots do contract longitudinally, a process that is enabled at least in part by deformable xylem elements known as wide-band tracheids.