RELATIONS OF ANIMALS AND PLANTS 131 



we only discovered its real home on our way back." 

 Here the bond between the animal and the plant 

 is mainly that the animal obtains shelter from the 

 plant, or from the sand accumulated by the plant ; 

 but we know from other sources that the " Djerd," 

 as well as other Gerbilles, are largely dependent 

 on the Limoniastrum leaves for food and water. 



A most interesting case of the same nature, but 

 one in which the animals obtain shelter, but not 

 food or drink, from the plant, has been studied in 

 Arizona and California, by GrinneU and other 

 American naturahsts. The Saguaro, or Giant Cac- 

 tus {Cereus giganteus\ is a picturesque and very 

 interesting inhabitant of those deserts (Fig. 41). 

 It is an upright cylindrical plant, with very few 

 branches, and covered with spines ; it attains a 

 height of 30 or 35 feet, and a diameter of 2 feet 

 6 inches. Its fruit is attractive to many species 

 of birds, and according to Tuomey " nearly half a 

 hundred birds feed upon the fruit of the Giant 

 Cactus, the Hst including all our thrashers, wood- 

 peckers, finches, and pigeons." But the Giant 

 Cactus is not interesting only because birds eat its 

 fruit, and as it harbours few insects one might sup- 

 pose that there would be no close link between it 

 and the birds for the great part of the year. A 

 connection however exists, and it is a very close 

 one. To quote from Grinnell : " It is clearly 

 apparent that the critical feature of the saguaro 

 which prescribes its avian dependants is nothing 

 less than the favourable opportunity offered for 

 the excavation of safe retreats in its trunk : and 



