"I 37 °"^ every single one of them is right 



mature trees have to be irrigated, though of course sagua- 

 ros which are growing nearby get no such pampering. 

 Measurement of the moisture lost by evaporation in the 

 course of a day makes the difference between the one and 

 the other no mystery. The date palm may transpire five 

 hundred quarts of water in twenty-four hours; a twelve- 

 foot saguaro as little as one-fiftieth of one quart, or approx- 

 imately one twenty-five thousandths as much. Obviously, 

 if you are a plant and are going to live in the desert, 

 leaflessness, succulence, a wax-coated skin and thorns to 

 reduce the circulation of desiccating air all pay off. 



But why should the saguaro adopt one device and the 

 queen of the night another? The two must be quite 

 closely related. It is not merely that both are cacti. They 

 actually belong to the same genus of the cactus family and 

 the structure of the stems v^th, in each case, longitudinal 

 ridges extending along their entire length, is similar. The 

 saguaro has taken advantage of this structure by making 

 the ridges serve as the high edges of the accordion pleats 

 which open to increase the volume when water is availa- 

 ble. The queen of the night takes only very minor advan- 

 tage of this feature and expands its root into a huge tuber 

 instead. 



The different devices adopted become even more inter- 

 esting when you realize that other plants, not at all related 

 and certainly having a very different evolutionary history, 

 have independently recognized the same possibihty of 

 which the queen of the night took advantage. A good ex- 

 ample is the common finger gourd of the desert, which 

 runs for long distances over the ground or clambers over a 

 shrub ff it can find one. 



Now the gourd family, or at least that genus v^th which 



