How Stingy Plants Hoard Their Water
NOT all plants in Arizona’s Saguaro National Monument are stingy with their water. In the coniferous forests of the majestic Rincon Mountains, the great bulk of the water is merely passing through, in at the roots and out through the leaves. But that is the remote area of the park. It is the hot, dry desert lowlands that attract the visitors. It is there that the water hoarders flourish, where less than 12 inches [30 cm] of rain falls in a typical year.
There are some 50 types of cacti in the park, but the one that hoards the most water is the namesake of this national monument, the giant saguaro, Carnegiea gigantea. The saguaro (pronounced “sawaro”) starts out tiny but ends up a giant. However, it takes its time getting there. The seed itself is no bigger than the period at the end of this sentence. By the end of its first year, the seedling may measure only one fourth inch [0.6 cm]. One foot [0.3 m] tall after 15 years, seven feet [2 m] after 50, its first branch comes at age 75. At this time, it begins to flower and produce seeds. When mature, one saguaro produces tens of thousands of seeds a year, some 40 million in a lifetime, only one of which may survive into old age. It may live up to 200 years, with a trunk two and a half feet [0.8 m] in diameter, be 50 feet [15 m] tall, and weigh ten tons—four fifths of which is water. And it’s very stingy with its water!
Also very grasping to get it! Its roots form a shallow network that spreads out up to 100 feet [30 m] in every direction. After a rainfall, they may soak up 200 gallons [750 L] of water, enough to last the saguaro a year. Cylinders consisting of 12 or more woody ribs run up the center of the trunk and branches to give strength. Accordionlike pleats allow it to expand or contract as water is stored or depleted. Its green, waxy skin performs photosynthesis and retains moisture. Its sharp spines discourage animals from stealing its water.
But the most amazing water-conserving mechanism of cacti is their ability to manufacture their food without excessive water loss. Photosynthesis—the process by which plants manufacture their food—requires water from the roots, carbon dioxide from the air, and sunlight. During the daylight hours, most plants transpire through the pores, or stomata, of their leaves an overwhelming percentage of their water supply, at the same time taking in the carbon dioxide and sunlight required for photosynthesis.
The cacti, however, cannot afford such water losses during the daylight hours of their hot, arid environment. Hence, they close the stomata of their stems to halt any water losses through transpiration. This, however, halts the intake of the carbon dioxide required for photosynthesis, which can only take place when there is sunlight to provide the needed energy. How is this dilemma solved? By a very unusual biological design.
The Dilemma’s Solution
Desert nights are cool, even cold. Cacti open their stomata at that time. They take in carbon dioxide but lose very little moisture through transpiration into the night air. But no photosynthesis takes place at this time. The carbon dioxide is stored by a totally different and most efficient set of chemical reactions, called the PEP system. Later, the carbon dioxide is released and sent to the location where the usual daytime processes of photosynthesis take place.
Photosynthesis itself is a very complex process involving some 70 separate chemical reactions and has been declared “truly a miraculous event.” The special manner in which cacti initiate it at nighttime to preserve water only adds to the miraculousness of it. Evolutionists, of course, say that it all evolved by blind chance, but since it is used by several unrelated plants, blind chance had to perform the miracle not once but many times. The evidence plus common sense indicates that it came about according to the design of an intelligent Creator.
A Servant of Many
The saguaro does community service. Beginning in late April and continuing into June, big bouquets of white blossoms form a covering cap over the tips of trunk and branches. Each individual flower opens at night and withers the next day. But each saguaro repeats the spectacle night after night for some four weeks, producing as many as a hundred blooms. The splashy display has earned it the honor of being Arizona’s state flower. Birds, bats, bees, and moths feed on the nectar and pollinate the flowers.
The fruit ripens during June and July, when javelinas, coyotes, foxes, squirrels, harvester ants, and many birds feast on the fruit and seeds. Flickers and woodpeckers excavate more nest holes in trunk and branches than they need, but the plant heals the wounds with protective scar tissue to prevent loss of water, and these cavities are later used by many other birds, including elf owls, screech owls, and small hawks. Competition is keen.
In years past, these gourdlike cavities were used by the Indians as water jars. The woody ribs that supported the huge weight of the water-laden saguaros were used to build shelters and fences. These green giants also provided a bounty of juicy figlike fruit, which the native Papago Indians knocked off the tops of trunks and branches with long poles. They made jam, syrup, and alcoholic drinks from them. The seeds were eaten by the Indians and their chickens. So important was the saguaro fruit to the Papago that the harvesting season marked the new year.
Desert plants are versatile in coping with the water problem. The mesquite plant gets all the water it needs. It sends a taproot from 30 to 100 feet [10 to 30 m] down to find an underground source. But how does the small seedling survive this long dry run until its taproot hits water? That’s just one of the unsolved mysteries of the desert. The night-blooming cereus grows a bulb that serves as its private underground reservoir. The creosote bush sends out far-reaching roots to collect water, which at the same time excrete toxins to kill any seedlings that start to grow near it.
Those beautiful annuals that bloom in the spring and carpet the desert with their colorful extravaganzas have none of these ingenious devices for surviving shortages of water. So how do they manage to do it? They avoid the shortages altogether! Their seeds have within them chemical inhibitors that prevent them from sprouting. A heavy rainfall will wash out these inhibitors, and the seeds will germinate and grow, the plants will bloom and produce seed for future plants. It takes at least a half inch [1.3 cm] of rainfall to remove the inhibitors; a light shower will not suffice. These seeds can measure rainfall, as it were, and unless enough has fallen to soak the ground sufficiently to see them through their complete life cycle, they lie dormant. They don’t start what they can’t finish.
The saguaros have interesting neighbors, do they not?
[Pictures on page 24]
The flowers and the fruit of the saguaro
[Picture on page 25]
They serve as high perches for hawks