FERTILE PLAINS AND DESERTS 351 



of it. Some plants, like the barrel cactus (Fig. 216), absorb large 

 amounts of water in times of rains and store it for months in 

 swollen stems and roots. They thus tide themselves over the 

 long drought which intervenes between rainy seasons. 



Desert plants are very different in structure and appearance 

 from plants in well-watered regions (Fig. 217). This is largely 

 because they have devices for safeguarding themselves against 

 the loss of water. All plants evaporate moisture through 



FIG. 217. Desert vegetation. 



their leaves ; a good healthy sunflower j>lant loses on a warm 

 dry day one pint of water by evaporation ; a large tree loses 

 barrels of water. Where rainfall is plentiful and ground 

 water is abundant this loss of water by evaporation is made 

 good by absorption from the soil. Where rain and soil water 

 are meager, as in deserts, there is no way of making good this 

 loss, and some means of preventing it is necessary. Desert 

 plants are constructed in such away that they get a large quan- 

 tity of water from the ground, lose little of it (Fig. 218), and 

 store much of it over the dry seasons. The mesquite has un- 

 usually long roots which take water from a large area ; the barrel 

 cactus through its swollen stem stores water in seasons of 

 plenty for use in seasons of drought. The ocotillo (Fig. 219) 



