272 READINGS IN BIOLOGICAL SCIENCE 



Flowers have developed with animals. Millions of years ago there were 

 no flowers. The first plants which "crept" from the ocean and freshwater on 

 to land had none. Then plants began to develop pollen and seeds which 

 contained stores of nourishment. The developing land animals soon took 

 advantage of these concentrated and rich foods, especially during dry or 

 cold periods when other foods were scanty. As animals formed habits of 

 visiting particular plants, the latter gradually "responded" and various mu- 

 tual benefits were derived from such associations. Finally the plants came 

 quite generally to offer food, and advertised the fact; in return, animals 

 carried pollen and distributed seeds. Fruits were developed which fur- 

 nished luscious food about a hard or inedible seed. Today, a robin which 

 swallows a cherry regurgitates the "stone," and thus these seeds are scat- 

 tered about where they may find favorable places for growth. Burrs and 

 little beggar-ticks take firm hold on the hairy coats of mammals and are 

 widely distributed. 



There are a variety of simple green plants, bacteria, and other fungi 

 which live in the bodies of animals and serve various beneficial functions 

 or cause diseases. Some of these aid in the digestion and assimilation of 

 food. In fishes certain luminescent bacteria live in special cavities and pro- 

 duce light when stimulated by the host in which they reside. There are 

 various fungi and bacteria which live in the skins of animals and cause 

 diseases. In man these are most frequent in dark-skinned races. There are 

 also bacteria which live within the body and cause diseases that are familiar 

 to all: typhoid, typhus, cholera, tuberculosis, leprosy, influenza, etc. 



Plants frequently furnish shelter or permanent homes for animals. The 

 fibrous and woody portions of land plants are especially suited for such 

 purposes. The paper-making v/asps chew up fibers and shape them into 

 nests which show characteristic forms and considerable architectural 

 complexity. In the tropics bromeliads, which are all more or less like pine- 

 apple plants, harbor a great variety of animals, some of which are greatly 

 flattened or otherwise especially adapted to live in the spaces between the 

 leaves. Leaf-rollers, web-worms, and certain ants fasten leaves together 

 to make nests. There are even a few insects that habitually take shelter 

 within pitcher plants, which entrap and devour most types of insects. 

 Natural or artificial cavities in trees are used as homes by bees, beetles, 

 woodpeckers, owls, hornbills, squirrels, and other animals. Wood, on ac- 

 count of its flexibility, strength and durability is an excellent material for 

 the dwelling places of animals. It is also a good insulator and therefore 

 protects animals against the extreme heat of summer and the cold of winter. 



Among the most interesting of the relations between plants and animals 

 are those furnished by ant plants. Most of these plants "provide" shelter and 

 food for their guests and in turn the ants protect them from browsing 

 animals and plant-eating insects. In America the ant plants are largely 

 acacias and cecropias. The former usually have hollow thorns at the bases 



