MAN'S NEED OF FOOD 



77 



Experiment 52. Do green leaves of plants manufac- 

 ture starch? 



First learn how to test for starch. Make a starch paste with 

 a pinch of starch and some water and put some of it into 

 a test tube. Mix a few drops of iodine solution with the 

 starch paste. What color is formed? Record the notes of 

 this experiment in your notebook. 1 This experiment is the 

 test for starch. 



Now obtain several leaves from a healthy plant which 

 has been exposed to direct sunlight for several hours. Boil 

 the leaves for a few minutes in water and then boil them 

 in alcohol until they are nearly white. The green matter 

 in the leaves will come out into the alcohol. Place the leaves 

 in water again and heat them gently until they are soft. Re- 

 move one of the leaves from the water and put several drops 

 of iodine solution on it. What color appears in the leaf? 

 What do you conclude from the experiment? Record your 

 notes in your notebook. 



OTHER INVESTIGATIONS WHICH YOU CAN MAKE 



1. Devise an experiment to show that gases are given 

 off by leaves of green plants. 



2. Devise an experiment to show that water is given off 

 to the air by leaves of green plants. 



3. Pull up a flowering plant and examine its roots, stem, 

 leaves, and flowers. Find out the function of each of these 

 parts. 



4. Devise an experiment to determine the parts of a stem 

 through which water rises from the roots. Suggestion: Use 

 a stalk of celery. 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



Why must we eat food? Every living creature from 

 the lowest to the highest form of life must have food. 

 Because of the great importance of food to plants 

 and animals and because of the many varieties of liv- 

 ing things many special methods have been developed 

 by them for obtaining food and disposing of it after 

 it is secured. In this unit of study we shall have time 

 enough only to see how food is obtained, prepared, 

 digested, and used by man. 



Scientists tell us that we must eat food for three 

 reasons. First, the human body must have a con- 

 tinual supply of energy to keep it working. It is like 

 a steam engine in this respect. You know that ia a 

 steam engine the burning of coal produces heat en- 

 ergy which generates the steam that runs the engine. 

 In the human body food is "burned" and furnishes 

 energy in the form of heat. Second, our bodies grow, 

 and foods supply the substances necessary for growth. 

 Third, the body must repair worn-out parts with 

 substances obtained from food. 



How do we grow? The human body is made up of 

 countless millions of tiny units of living matter called 

 cells. (See Fig. 118.) If a compound microscope is 

 available in your school and you wish to see a cell, 

 carefully scrape some material from the inside of your 



1 See workbook, p. 29. 



cheek, place it on a glass slide, add a drop of water, 

 and examine it under the microscope. 



Each individual cell helps to do the work of the 

 body. It receives the digested food, changes it into 

 living matter, and grows. When the cell reaches a 

 certain size it divides and forms two cells. When these 



Five sided 

 liver cells 



Bone. te\U 

 surrounded 

 taoney 



"muscle cells 

 from the 

 stomach 



Tat cells 

 The protoplasm 

 and nucleus v><we 

 toee.n pushed \o 

 one side, tov t\^e 

 '? f<xt de.po%\Te,d in 

 'snearhti the cell, 

 nerve and 

 -nerve 

 endrogs 



Cells in connective tissue. 

 The cells < fi\ve out a jelly-like 

 material which hardens to 

 ci strong elastic mass. 



FIG. 118. CELLS OF THE HUMAN BODY 



two cells have grown to the same size as their parent 

 cell they also divide (Fig. 119). It is by this constant 

 dividing of cells that our bodies increase in size, and 

 the life of the human body is the combined life of its 

 individual cells. 



Why do we need energy? Our lives are made up of 

 activities. During the daytime we may walk, run, 

 jump, play, or work. At night when we are asleep we 

 are more quiet, but even then our heart is pumping 

 blood to all parts of the body. Our chest rises and 

 falls as we breathe. Muscles of our digestive system 



FIG. 119. THE DIVISION OF CELLS 



are at work aiding the process of digestion. The food 

 that we eat is oxidized by the oxygen in the air that 

 we breathe, thus providing the necessary energy to 

 our bodies in the form of heat. Our bodies remain 

 constantly at a temperature of 98.6 F. when we are 

 well. When the temperature of our body rises or falls 

 even one degree from this point the variation is an 

 indication of sickness. 



What are the sources of our food? By looking care- 

 fully over the list of foods that you eat you will notice 

 that they can be grouped first of all into two classes : 

 those that we obtain directly from plants and animals, 

 and those like water and salt that we get from non- 

 living things. The foods from plants and animals are 

 called organic foods; the foods from non-living things 

 are called inorganic foods. 



Most of our foods are of the organic type, that is, 



