150 FROM FISH TO PHILOSOPHER 



and even in summer may approach the freezing point. 

 Only the superficial earth, however, suffers these ex- 

 tremes, and at a depth of 6 inches the daily fluctuation 

 in temperature is reduced to a quarter of that of the 

 air; at 10 inches, to a thirtieth. Because the desert 

 temperature falls rapidly, the dew point is frequently 

 reached within an hour or so after sunset, and the dew 

 that accumulates on plants and grasses is sometimes a 

 significant source of water for many animals. There is, 

 however, considerable variation in the humidity of desert 

 areas— the formation of dew in the Arizona desert, which 

 is protected by mountains, is a rare event, whereas the 

 annual precipitation of dew in the Negev of southern 

 Israel, which receives moisture-laden winds from the 

 Red Sea and the Mediterranean, amounts in the average 

 to 8.7 inches, or more than many deserts receive as rain. 

 Desert plants fall into three general groups. The an- 

 nuals sprout quickly in the rainy season, bloom in an 

 incredibly short time and then wither rapidly after re- 

 seeding the soil. It is these ephemeral annuals that give 

 to the desert its many colored flowers for a short period 

 of the year. The second group is represented by peren- 

 nials that have a bulb, tube, or fleshy root buried fairly 

 deeply in the soil; these sprout and multiply dining the 

 rainy season, though less rapidly than do the annuals. 

 Neither of these two groups store significant quantities 

 of water in the fleshy parts. The third and most impor- 

 tant group, from the year-around point of view, is repre- 

 sented by the succulent plants such as the cacti of the 

 Western Hemisphere or the spurges (Euphorbia) of the 

 Eastern Hemisphere. In both, the stems are greatly en- 

 larged and leaves are small or absent; stems and leaves 

 are tough and leathery and the thickened cuticle may 

 be covered with wax, resin, or densely matted hairs, and 

 in all cases the enlarged stems act as reservoirs for the 

 storage of water. A large tree cactus might contain as 

 much as a thousand gallons, and a donkey can be wa- 

 tered with the juices of the giant cactus, Cereus gigan- 



