CHAPTER XIII 

 LIFE ON THE EAETH 



Plants and Animals. Plants and animals are com- 

 binations of the earth's elements endowed with life. By 

 means of the sun's energy they are able, the plants directly 

 and the animals indirectly, to do both internal and external 

 work which results in growth, reproduction, and other ac- 

 tivities. Since plants and animals are entirely dependent 

 upon the earth and sun for their existence, they, like other 

 earth and sun phenomena, should be studied in this course. 



Plants. Although in their lower microscopical forms it 

 is very difficult to distinguish between plants and animals, 

 yet the forms ordinarily seen differ greatly. Most plants 

 are fixed and consist of root, stem, and leaves, while most 

 animals are movable and possess a variety of different parts. 

 But some plants, like the seaweeds, appear to have no roots ; 

 some, like the dandelion, no plant stem, and some, like the 

 cactus, no leaves. 



If we dig around the base of a tree, we find in the soil a 

 network of roots holding firmly erect a pillar-like stem with 

 branches bearing a profusion 6f leaves. If we examine these 

 divisions carefully, we shall find that each has a distinct part 

 to play in the life work of the tree. We shall also find (1) that 

 plants as well as animals need air, water, and other kinds of 

 food, (2) that plants and animals take in, digest, and assimi- 

 late food, and (3) that each in the higher forms has parts 



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