Again, as might be expected, the cactus is a highly tuned water preserving organism. The problem is that first these highly tuned machines must access water they need. Everything you see above ground on a cacti is designed to hold water in. The “skin” is thick and contains a waxy layer on top and spines prevent animals from eating the plant to obtain water. No water enters the cactus from above ground and only the minimal amount necessary is allowed to escape out of the cacti. Only roots do the job of zealously drinking the latest rainfall. Nearly all cacti have very shallow root systems, most of which only penetrate no more than one foot underground. A few cacti such as the saguaro have taproots that penetrate as deep as three feet into the soil. Even that isn’t very deep compared to some desert shrubs such as creosote brush with roots extending over six feet in depth and Ironwood over 30 feet deep. But while cacti lack depth of roots they have a vast breadth of shallow roots. I once examined the root system of a saguaro cactus where most of the soil had been eroded away on one side of it yet left the most of the root system intact and free from soil. The saguaro’s roots extended twenty plus feet away from the cactus and I couldn’t find a single root deeper than 24 inches into the soil, most were about one foot deep. Others have traced the saguaro’s root system as far as 50 feet from the trunk. Most other cacti also have extensively shallow root systems.
Initially, it seems counter intuitive for desert plants to have shallow roots until you realize that only rarely does moisture from rain in the desert penetrate more than two or three feet deep. Then, once it does rain slumbering cacti roots awaken within hours, sending out tiny rootlets that quickly absorb water. And when a cactus drinks, it doesn’t just sip, it binges by drinking as much as it can as fast as it can. In hot deserts roots can’t grow too close to the surface simply because extreme soil temperatures kill the roots, so roots begin about an inch below the surface. In cooler regions, such as in Colorado, the soil surface doesn’t get extremely hot so cacti roots can grow just millimeters below the surface, greatly increasing their ability to take advantage of every little rainfall. A scientist once found prickly pear cacti in Colorado with roots only two millimeters below the surface. This cactus was able to survive on only two millimeters of rain a year owed to the fact of its extremely shallow roots!
Once wet soil activates binge drinking, the cactus absorbs way more water than it is currently using so it must store it somewhere. Tissue within the stems, pads, or joints absorbs this water and stores it for later use. As more water is absorbed the stem, pad or joint swells. Simply by looking at a cactus you can tell how well watered it is. If it is swollen it is well watered and good to go for months without rain, even in hot weather. If it is skinny or looking shriveled the cactus needs water. Cacti are well adapted to swelling and shrinking. Saguaros and other columnar cacti have palliated or accordion shaped stems that readily expand when swollen with water or compress when water is depleted. A mature Saguaro can hold thousands of pounds of water by simply expanding its accordion shaped skin. Pads of prickly pears and joints of cholla, also swell and contract readily, easily doubling or more in size when well watered.
After the desert soil dries out, roots begin to go dormant. Tiny rootlets that developed in the wet soil dry out and die. Wet mucilage around the roots also dries out and forms a sheath surrounding the roots preventing the roots from directly contacting the soil. When this happens, the cactus quits drinking or even attempting to drink water from the soil. In-fact, the sheath functions as a barrier to prevent any water from leaving the cactus. This is actually quite unique to the plant world being most plants require their roots to be in contact with at least a little moisture in the soil at all times. Once the soil dries out however, the cactus must draw from the reserves of water it has stored up in its tissue in order to carry out photosynthesis. Even so, after a good rainstorm a cactus can absorb enough water to carry it through many months of intense drought.