USE AND STRUCTURE OF A FLOWER 31 



they feed, while others tunnel through the wood, making homes 

 there. Spiders build webs on plants, often using the leaves for 

 shelter. Birds nest in trees, and many other wild animals use 

 the forest as their home. Man has learned to use many kinds of 

 plant products to aid him in making his home, wood and various 

 fibers being the most important of these products. 



What Animals do for Plants. — So far it has seemed as if green 

 plants benefited animals and received nothing in return. We shall 

 see later that plants and animals together form a balance of life on 

 the earth and that each is necessary for the other. Certain sub- 

 stances found in the body wastes from animals are necessary to the 

 life of a green plant. 



Insects and Flowers. — Certain other problems can be worked 

 out in the fall of the year. One of these is the biological inter- 

 relation between insects and flowers. It is easy on a field trip to 

 find insects lighting upon flowers. They evidently have a reason 

 for doing this. 



The Use and Structure of a Flower. — It is a matter of common 

 knowledge that flowers form fruits and that fruits contain seeds. 

 Flowers, then, are not merely things of beauty, but are very impor- 

 tant parts of plants. On our field trip we saw many flowers and 

 noticed that they are of various shapes, colors, and sizes. It will 

 now be our problem first to learn to know the parts of a flower, 

 and then to find out how they are fitted to attract and receive 

 insect visitors. 



The Floral Envelope. — The expanded portion of the flower 

 stalk, which holds the parts of the flower, is called the receptacle. 

 The green leaflike parts covering the unopened flower, when 

 taken together, are called the ca'lyx. Each of these parts is a 

 se'pal. The more brightly colored structures are the pet'als. 

 Together they form the coroVla. The calyx and corolla together 

 are called the floral envelope. 



The corolla is of importance in making the flower conspicuous. 

 Frequently the petals or corolla have bright marks or dots which 

 lead down to the base of the cup of the flower, where a sweet fluid 

 called nectar is secreted by nectar glands. It is principally this 

 food substance, later made into honey by bees, that makes flowers 

 attractive to insects. 



