DISTRIBUTION AND ADJUSTMENT 



99 



plant may be thick and succulent, storing up water in time of 

 plenty to use in time of drought. The common cactus of the 

 dunes is a good illustration (Fig. 60). The plants growing where 

 moisture is lacking often develop an extensive superficial root 

 system to gather up the dew and the showers before the water 

 has time to dissipate or sink deep into the parched soil. One 

 can pull out fine stringHke roots in the open dunes that run just 

 below the surface for hundreds of feet to the tree or bunch grass. 



Fig. 61. — The six-lined lizard, Cnemidophorus sexlineatus 



Many of the animals of the open dunes are in hiding during 

 the day, some of them like the burrowing spider, Lycosa wrightii, 

 in holes that run down to the cool and moist soil layers. The 

 surface of the sand is covered in the early morning with the fresh 

 tracks of many animals that have been out during the cool 

 of the night to satisfy their needs, while the same areas are 

 apparently uninhabited by day. The six-lined lizard (Fig. 61), 

 that like the cactus is a desert form left in this sandy oasis 

 by the lake, excretes its uric acid in solid form, a conservation of 

 water common to many reptiles. 



