Winter Showers Bring Desert Flowers—And Also Refill Plant Reservoirs
SOME desert seeds seem to measure rainfall. They will not sprout until a half inch (1.3 cm) or more falls. They can be thoroughly soaked with less, but they will not germinate. They also seem to know the direction from which the water comes. If enough comes from above, they will sprout; if it soaks up from below, they refuse to do so. They’re not just being fussy; they’re very wise. These wise seeds belong to the flowering annuals that in springtime may carpet the desert with dazzling colors.
But how do they measure rainfall? At times certain salts are in the desert soil, and the seeds refuse to sprout in their presence. The salts are soluble in water. Light showers may soak the seeds but won’t leach out the salts. It takes frequent and heavy showers to dissolve the salts and carry them deeper into the soil, away from the seeds. And the rain must soak into the soil from above; water soaking up from below may dissolve the salts but won’t carry them away.
Sometimes the problem is not with the soil but with the seeds. In the covering of some desert seeds, there are water-soluble chemicals that inhibit germination. A light rain may soak the seeds, but it takes several heavy rainfalls to remove all the troublesome chemicals. Some inhibiting substances in the seed coverings are not removed even by heavy rain; it takes the action of certain bacteria. But these bacteria only do their job when there is prolonged moisture in the seed. So again it is lots of rain that is required.
Why are the seeds of desert flowers so persnickety about all of this? If they sprout and start to grow at the first light shower, their roots will find no water deeper down. The hot desert sun will scorch the plants before they flower and bear seed. But if the seeds are made to wait until the ground is soaked deep down, their roots will find moisture even when the surface soil is dry.
So it is for survival that the salts in the soil make the seeds wait until heavy rainfall has leached them out. The germination inhibitors in the seed coverings perform the same service. Other substances in the seed coatings prevent sprouting, but they are removed by bacteria that don’t do their work until rain has soaked the seeds. By these various means, seeds wait for repeated heavy rainfalls before germinating.
When copious winter showers do not come, neither do the deserts blossom like the rose. But when they do, springtime in the desert bursts forth with pageants of color that bring oohs and ahs from the flocks of visitors that come from miles around. And should not these admiring throngs show gratitude to the Creator, who built this wisdom into these seeds and who sends the winter showers that bring forth the desert flowers?
In all of this, there is a moral for us. When the seedlings of these desert annuals come up, there may be thousands per square yard. They do not kill one another off—none of this ruthless evolutionary “survival of the fittest” business! They adapt. Each one grows smaller, demanding less, sharing space and water. In one small area, 3,000 plants were found, belonging to 10 different species. Each one had at least one flower and produced at least one seed. If people are so much wiser than flowers, why can’t the different races live together and share?
Plants That Maintain Reservoirs
Then there are the succulent cacti that survive the desert’s long dry spells by storing up water on the rare rainy days. Some use underground containers, while others hoard it in thick stems. For these green stems to absorb carbon dioxide and perform photosynthesis, the stomata, or breathing holes, must be kept open. Yet this invites disaster, as precious water then escapes in the form of water vapor. The loss is minimized by the stomata’s remaining closed during the daytime heat, opening up only during the cool nights. Moreover, in the desert cacti, the stomata are sunk beneath the stem surface in depressions, which further limits water loss.
The sparse desert rainfall seldom penetrates very far below the surface, so cacti roots are usually shallow and spread out over a big area to suck up as much water as possible. As their internal reservoirs fill up, the plants swell, and as the water is used up during dry spells, they shrivel. In many such plants, leaves have been reduced to spines, which also ward off predators that come to eat or drink.
The most striking of this desert community is the giant saguaro. It reaches an age of 200 years, a height of 50 feet (15 m), a weight of 10 tons, and is four fifths water. Its bulk presents relatively little evaporating surface and is fluted like an accordion—allowing it to expand or shrink as water is added or used up. This pleated surface also eliminates large flat areas exposed to the direct rays of the sun and actually shades itself.
Finally, a glorious gift these desert cacti bring to their surroundings each year is a profusion of brilliantly colored flowers. So just as do the poppies and other springtime annuals lured into glorious displays by heavy winter rains, every year these water-hoarding perennials contribute to the desert’s blossoming like the rose.
[Pictures on page 16, 17]
Golden poppies, blue lupines, and giant saguaros in Arizona
Cow’s tongue cactus
[Pictures on page 18]
Cactus wren on blooming saguaro
Blossoms, usually on the tip, cover this saguaro