Summer World_ A Season of Bounty - Bernd Heinrich [75]
We saw no sign of free water. Yet during World War II two German geologists from nearby Windhoek—Henno Martin and Hermann Korn, with their dog, Otto—managed to hide out here undetected for two and a half years (to avoid being put in an internment camp). They lived like Robinson Crusoe during those years, and Martin later wrote a book about their experiences. In it, he describes the effect that water has on life in the desert. Martin and Korn had experienced a drought in the Namib for several years, and one night they heard thunder:
I had never before in my life heard [such thunder], or experienced such a cloudburst—and now the scorched and battered life began to raise its head again—within hours, bushes that had looked dead began to show shoots of green and in the shade of rocks ferns began to unroll delicate light green leaves. The desert was alive everywhere: seeds that had lain dormant for years came to life and pierced the crust of the earth; almost overnight the balsam bushes covered themselves with green leaves like young birches; the first yellow flowers opened their petals to the sun, and once again we found the speckled eggs of the quail amidst the grass and stones; and the lukewarm water on the pools swarmed with little crab-like insects whose eggs had survived the years of dryness and scorching sunshine.
Adaptations of plants to deserts include dormancy and a variety of structural and behavioral adaptations. The majority of desert plants depend on a strategy that capitalizes on small size. They are annuals that spring up from dry, dormant, heat-resistant seeds. Some of these seeds may wait up to half a century before they are activated. The plants’ challenge is to be quick enough to respond to rain so that they can produce their seeds before the earth dries up again, while not jumping the gun to start growth until there is sufficient water for them to grow to maturity for seed production. Some achieve this balance on a tightrope by “measuring” rainfall. They have chemicals in their seeds that inhibit germination, and a minimum amount of rain is required before these are leached out. Others have seed coats that must be mechanically scarred to permit sufficient wetting for germination, and the scarring happens only when they are subjected to flash floods in the riverbeds where they grow. A plant in the Negev Desert releases its seed from a tough capsule only under the influence of water through a mechanism that resembles a Roman ballistic machine. Its two outer sepals generate sideways tension that can fling two seeds out of the fruit, but the two seeds are held inside by a lock mechanism at the top. However, when the sepals are sufficiently wetted, then the tension increases to such an extent that the lock mechanism snaps, and the capsule “explodes” and releases the seeds (Evenari et al. 1982).
In moist regions where it rains predictably (though not necessarily in abundance), we help agricultural plants to capture the precipitation by scarring the soil to facilitate the infiltration of the water into it, and hence into the roots. Least runoff and maximum water absorption are achieved by plowing the soil. However, such a strategy would not work in a true desert such as the Negev. A different program is required there because rain is infrequent and plowing would facilitate only the evaporation of scarce water from the soil. The solution applied by the peoples who inhabited the Negev in past centuries was a practice they called “runoff farming.” Farmers had mastered harnessing the flash floods that rush down into the gullies by catching the runoffs—not only by making terraces but also by building large cisterns into which the water was directed to be held for later use. Remnants of these constructions still exist.
Water-storage mechanisms have been invented by other organisms living in deserts, but mainly through modifications of body plan. Many plants, especially