Louise Jawos, age 12, of St. Catharines, Ontario, Canada, for her question:
How can cactus plants live in the waterless deserts?
If the regions we call deserts were totally waterless, cactuses could not grow there. But nearly every spot on the earth gets a certain quota of rainfall. Deserts get less than their share. A few are too dry to support even cactuses, though an assortment of wind blown seeds waits hopefully in the arid soil.
The plant world asks very little of the earth and its basic needs are found almost everywhere. It needs sunshine and moisture and crumbly soil that contains a diet of certain chemicals. As a rule, we have to fertilize our farmlands to replace the plant food chemicals used by our yearly crops. Sometimes we have to irrigate with extra water to help our cultivated plants to do their best. But in the wilds, plants adjust themselves to the limits of various regions. The plant kingdom happens to have many remarkable qualities. For one thing, it has a stubborn determination to survive and expand to occupy every spot of useable ground. Plants strive hard to make the very best of hardships and limitations.
When necessary, nature helps them to change and adjust themselves to conditions that seem impossible. These adjustments work slowly, very slowly, and the name of the game is selection. Rushes and reeds need lots of water. If their seeds are blown into arid deserts they fail to take root and grow there. But some of their grassy cousins are built to get along with less water. Not all the plants of our deserts and arid prairies are cactuses. Many of them are grasses and scrubby bushes. But the amazing cactuses manage to survive and even thrive in even more impossible regions.
There are hundreds of different cactuses ranging from little plants to tree sized giants. All of them have special qualities that enable them to make the best of arid conditions. Desert showers are rare and sudden and the deluges of rain rush away to drain off down sandy washes. Water may saturate the surface sand but little of it sinks down to be stored below the ground. Cactuses have roots that spread out just under the surface, all ready to soak up every possible drop of shallow water from the sudden showers. Even the Joshua trees and giant saguaros do not poke their roots deep into the arid ground below the surface.
Special surface roots enable the cactuses to soak up the sudden showers. But this water must be stored for weeks, months, or perhaps years until the next desert deluge. And the cactuses are built to do just that. Their tissues are riddled with large reservoir cells. After a shower, these pockets are filled with water supplies to last a long, long time. What's more, the cactuses have special devices to pre¬vent their stored water from evaporating and being stolen. Other plants evaporate water extravagantly through pores in their fragile leaves. The so called leaves of the average cactus are actually modified stems covered with thick skins to seal in the water. And the average cactus also has a quota of spikey prickles to discourage thirsty desert animals from stealing its precious stores of the scanty rainfall.
Sometimes, desert showers occur unexpectedly at any time of the year. Other deserts can expect most of their showers during a certain season. In any case, the ingenious cactuses are prepared to make the best of the good times. After a rain, all sorts of new plants sprout from their seeds waiting in the arid ground. The established cactuses add new growth and blossoms and few other plants can compete with the gaudy blossoms of the cactus clan. After a spell of spring rains, California's vast Mojave Desert blossoms forth in a rainbow expanse of gorgeous cactus blooms, crowded almost cheek to cheek.