SUMMARY 233 



asexual and sexual. The ordinary growth processes of plants 

 and animals, those employed by the agriculturist, in slipping, 

 layering, budding, and grafting, and the process known as re- 

 generation in animals, are examples of growth through cell divi- 

 sion and multiplication. Cell division, budding, and the growth 

 of spores, are methods of asexual reproduction of animals and 

 plants. But another kind of reproduction also takes place. From 

 almost the simplest forms of plants and animals to man, sexual 

 reproduction is found. In its simplest form it is merely the fusion 

 of two cells to form a new cell, which in plants is called a zygote 

 and in animals a fertilized egg, and which under favorable condi- 

 tions will develop into a complete plant or animal. 



The life history of some organisms is a very simple story indeed ; 

 think of pleurococcus. But most plants and animals have com- 

 plicated life histories. Some include a change of form or meta- 

 morphosis, and some include (as in the liver fluke) parasitism and 

 metamorphosis. The higher in the scale of life, the fewer eggs (as 

 a rule) are produced and the greater is the protection of the eggs 

 and young. In mammals this protection is greatest, for the eggs 

 are held in the body of the mother until the embryo is developed 

 considerably, when it is born and cared for by the mother. 



Problem Questions 



1. Why do cells have to divide when they reach a certain size? 



2. What kind of development is fission ? Explain. 



3. What kind of development is conjugation? Explain. 



4. Compare asexual and sexual spores in development. 



5. Explain the process of fertilization in flowers. 



6. Compare complete and incomplete metamorphosis. 



7. How might metamorphosis be of value to an animal? 



8. Explain alternation of generations. 



Problem and Project References 



Hunter, Laboratory Problems in Civic Biology. American Book Company. 

 Gager, Fundamentals of Botany. P. Blakiston's Son and Company. 

 Hegner, Introduction to Zoology. The Macmillan Company. 

 Holmes, Biology of the Frog. The Macmillan Company. 

 Morgan, Experimental Zoology. The Macmillan Company. 

 Needham, General Biology. Comstock Publishing Company. 

 Shull, Principles of Animal Biology. McGraw-Hill Book Company. 

 Transeau, General Botany. World Book Company. 



