88 The Giant Saguaro

The giant saguaro may well symbolize the desert, but it is more than a symbol, even more than a plant. To a host of creatures—birds, mammals, and insects—the stately saguaro is home. It is an apartment house—and an air-conditioned one at that.

A pair of Gila woodpeckers once constructed an apartment atop a twenty-five-foot saguaro. These carvers cut a hole two inches in diameter into the cactus. They bored straight for about three inches, then turned sharply downward and dug a chamber about nine inches deep and four inches wide.

White pulp exuding sticky sap lined the hole. Over the next few months, the sap hardened to form a dry, tough callous all around the hole, making the hard walls and a floor for the apartment. In April, the birds came back to move into their apartment and get started on the important business of raising babies. On the hard, bare floor of the hole, the female laid four white eggs and sat on them in cool comfort.

Within the plant, behind the tough lining, the living sap of the cactus flowed. This liquid kept the temperature inside the hole well below the hot midday temperature outside. All day the cactus absorbed heat from the sun, but the interior of the hole remained cooler than the sweltering outdoors. At night, when the outside heat quickly escaped into the sky, the woodpeckers were snug in their house, heated now by the warmed cactus sap.

88 巨型树形仙人掌

巨型树形仙人掌很可能是沙漠的象征，但它不仅是一个象征，甚至不仅仅是一种植物。对于许多生物——鸟类、哺乳动物和昆虫来说，高大的树形仙人掌就是家园。它是一栋公寓楼——而且是带空调的那种。

一对吉拉啄木鸟曾在一棵25英尺高的树形仙人掌顶部建造了一个“公寓”。这些“雕刻师”在仙人掌上凿了一个直径2英寸的洞。它们先笔直地凿了大约3英寸深，然后急剧向下转弯，挖了一个约9英寸深、4英寸宽的洞穴。

渗出黏糊糊汁液的白色果肉铺满了洞口。在接下来的几个月里，汁液变硬，在洞口周围形成了干燥、坚硬的痂，构成了公寓的硬墙和地板。4月份，鸟儿们回来搬进了它们的公寓，开始了养育幼鸟的重要工作。在洞穴坚硬、光秃的地板上，雌鸟产下了四枚白色的蛋，在凉爽舒适的环境中孵蛋。

在植物内部，坚硬的衬层后面，仙人掌的活汁液在流动。这种液体使洞穴内部的温度远低于外面炎热的正午温度。一整天，仙人掌都在吸收太阳的热量，但洞穴内部仍然比闷热的室外凉爽。到了晚上，当外面的热量迅速散发到空中时，啄木鸟们在它们的房子里很舒适，现在是被温暖的仙人掌汁液加热的。