Green plants (Chapter II.) are quite independent of all other forms of life, and can build up their substance from water, mineral matter, and the carbon dioxide of the air. The taking in of carbon dioxide and giving out of oxygen by green plants has nothing whatever to do with [Pg 244] respiration; it is part of their process of feeding. Green plants breathe in the usual manner—by taking in oxygen and giving out carbon dioxide. It should, however, be noticed that the peculiar method by which a green plant obtains its carbonaceous food is of the highest importance to animal life; for by this process the amount of injurious carbon dioxide in the air is considerably lessened, while the proportion of life-supporting oxygen in it is greatly increased. Fungi (Chapter XI.) seem to be intermediate, as regards their method of feeding, between green plants and animals. They require their carbonaceous food in an organic form, that is, already prepared by other living things; but they can obtain the other elements of their food from mineral salts and water. The thoughtful student will be increasingly impressed by the extent to which the plant and animal kingdoms are dependent upon each other, and by the manner in which each utilises the waste products of the other for carrying on its own life processes. EXERCISES ON CHAPTER XIII. 1. Of what parts does the skull of a rabbit consist? What is the use of each part? 2. Describe one of the long bones of a rabbit’s leg. To what features does it owe its strength? 3. The bones of the skeleton are useful (1) as affording points of attachment for the muscles; (2) as affording protection for delicate tissues and organs. Give examples of each of these uses. Do not give the technical names for the various muscles. (King’s Schol., 1902) 4. Draw and describe one of the middle joints of the backbone of a quadruped, and explain the uses of the various parts. (1898) 5. What is meant by “digestion”? Why must food be digested? Where does digestion take place? 6. Give full practical instructions for demonstrating the chief properties of saliva, and its action upon various kinds of food. (1897) [Pg 245] 7. Prove that the action of human saliva upon starch is not due to living particles contained in it. (1898) 8. What are the chief uses of the blood? Why is it necessary that it should be kept in motion? (1901) 9. Where would you look for the Aorta in a sheep’s heart? What valves are found in it? How does it differ in appearance and feel from a large vein? (1901) 10. What kinds of valves are found in a sheep’s heart, and where are they placed? Describe a valve of each kind. (1898) 11. How does air breathed out from the lungs differ from common air? How can the differences be demonstrated? (1898) 12. Describe the process by which a mammal renews the air in its lungs. 13. What is meant by “respiration”? Why is respiration necessary?