leaves. Less is known, however, of the forces which cause the sap to descend than of those which produce its ascent, but it is known that it descends through the large outer cells that, in the tree, constitute the bark. This is why removing its bark will kill a tree. Source of salts in the soil.—It was stated in a preceding paragraph that plants got their nitrogen and mineral supply from the soil. This being the case, the soil must be constantly losing its supply of such matter and it must be as constantly renewed if plants are to grow in it. This is done by the use of fertilizers. Many compounds of the salts necessary for fertilizing the soil are now prepared by the chemist, but, until recently, animal excreta and decaying vegetable matter were the only fertilizers to be had and they are still very much used. To understand how animal excreta can supply the soil with mineral matter it is necessary to realize that animal excreta contain not only salts eaten in food, but also, especially the urine, salts formed in the body by chemical reactions occurring in metabolism. The protein in plants and in animal excreta does not exist in the form of nitrates, but when decaying vegetable matter and manure are spread over the ground they, by the influence of bacteria, undergo decomposition, and their protein matter is slowly formed into ammonia compounds, some of which, uniting with oxygen, and in other ways, are broken down to nitrates and other nitrogen salts which are capable of going into solution in the soil water and being absorbed by plants. Another way in which bacteria supply plants and the soil with nitrogen is that various species are able to absorb nitrogen from the air and some of these species enter the roots of such plants as peas, beans, clover, alfalfa, and they live and multiply in little tubercles that develop on the roots in consequence of the invasion of the plants by these organisms. The bacteria, as they develop in these nodules, in some unknown way, combine the nitrogen they absorb so that it is available for plants. It is thought that perhaps the bacteria after absorbing the nitrogen secrete a nitrogenous substance, some of which is absorbed by the plant cells while a portion osmoses into the soil. The nitrogen supply of the soil in which such plants grow is thus increased, instead of diminished, as it is when used for the growth of other plants. The soil nitrogen supply is still further increased when the roots of such plants are allowed to rot in the ground. The food cycle.-As shown in the preceding pages, plants and animals, including man, are absolutely dependent upon each other for their sustenance, and there is a continual cycle in progress by which one. form of life supplies the other with matter for their life and growth. Animals eat plants and, when these are oxidized in their bodies, they eliminate CO2 and other waste matter that plants can use for their food and development. Man supplies CO', also by burning plants, as wood, and plant products, as coal, oil, and gas. Effect of plant life upon the atmosphere.-Plants not only supply man and other animals with food, but they also keep the air pure by extracting CO2 and supplying oxygen. One source of the oxygen supply can be seen in the equations on page 301. Oxygen is freed also in the making of fats for, as can be seen in the following formulas of some of the simple fats, there is less oxygen in fat than in sugar and starch: Stearin-C3H5(C18H35O2)3 In some metastasis processes CO2 is liberated; therefore plants eliminate a small amount of CO2, especially at night when photosynthesis is not taking place. CHAPTER XIX NATURE AND NUTRITIVE VALUE OF SOME OF THE COMMON FOODS Classification of Foods-Nature, Digestibility, and Nutritive Value of the more Common Foods and Beverages— Nature and Action of Condiments and Spices. Animal Foods THE more common animal foods include the flesh of mammalia-generally spoken of as meat—-birds, fish and other sea-food, eggs, milk, and honey. Nutritive value of animal foods.--With the exception of honey, milk, liver, and shellfish, there is little or no carbohydrate found in animal foods. The reason for the absence of carbohydrate in meat is that that which is present in the living animal is drained off with the blood when the animal is slaughtered, or else changed to lactic acid and other products of the decomposition of glycogen and glucose shortly after the death of the animal. Consequently the only solids provided by the majority of animal foods are proteins, fats, and mineral matter. Animal foods are usually a more expensive form of nutriment than vegetable foods, but, as can be seen. by looking at the table on pages 340-344 they contain more protein than other foods, and animal protein is more thoroughly digested and absorbed than vege 3 8 7 2 4 1 11 9 5 FIG. 61. CUTS OF BEEF. I, neck; 2, chuck; 3, ribs; 4, shoulder clod; 5, fore shank; 6, brisket; 7, cross ribs; 8, plate; 9, navel; 10, loin; II, flank; 12. rump; 13, round; 14, second-cut round; 15, hind shank. table protein. Also, the flavor of meat, the stimulating effect of the meat extractives on the glands which secrete the gastric juice, the many uses of eggs and milk, all make animal foods very desirable articles of diet for the majority of people. Comparative digestibility of animal foods.--The following list of the comparative digestibility of common animal foods is one to be found in several books on dietetics. The foods are mentioned in the order of their digestibility, beginning with the most digestible. |