RELATIONS OF ANIMALS AND PLANTS 133 



in which the Giant Cactus grows, and takes a dis- 

 used woodpecker hole for its nesting-place. It is 

 therefore dependent for its very existence on the 

 presence of a particular cactus, and either of two 

 particular woodpeckers. Less strictly dependent 

 upon the cactus and woodpeckers are a Screech 

 Owl (Otus asio gilmani), a Sparrow Hawk {Falco 

 s'parverius), a Flycatcher (Myiarchus c, cinerascens), 

 and other birds. None of these are confined to 

 the area inhabited by the Giant Cactus, but they 

 all inhabit that area, and within it they aU use old 

 woodpecker holes as nesting-sites. 



Frequently a single trunk of the Giant Cactus 

 contains nests of one or other woodpecker, and also 

 of one of the birds which use the old woodpecker 

 holes. Honey-bees also use these excavations as 

 hives. One must remember that the number of 

 Hving creatures which eventually depend upon the 

 Giant Cactus includes the scavengers in the birds' 

 nests and bees' nests, the insects, few though they 

 may be, which devour it or frequent its blossoms, 

 and many others. AU these organisms depend upon 

 the growth of Cereus giganteus for their existence in 

 certain areas. 



There are instances in which, contrary to the 

 general rule, the plants are dependent upon the 

 animals, and although these cases are not very com- 

 mon in deserts, a few may be mentioned. Botanists 

 teU us that there are two important agents, the 

 wind and the insects, by which the pollen of plants 

 is carried to the female flower, or female part of the 

 flower, which it is intended to fertiUze. In many 



