144 



All Food 



leaves in bright sunlight. At night photo- 

 synthesis stops. Then the concentration 

 of oxygen inside becomes less than out- 

 side, and, consequently, oxygen dif- 

 fuses from the surrounding atmosphere 

 through the stomata into the leaves and 

 into the cells. 



Now in respiration carbon dioxide is 

 made. In the daytime it stays inside and 

 at once combines with water in the 

 process of photosynthesis. But at night 

 the carbon dioxide produced in respira- 

 tion is not used up by the cells and car- 

 bon dioxide passes out of the cells. Note: 

 Though respiration goes on all the time, 

 it is only at night that oxygen enters the 

 plant and carbon dioxide leaves. 



Now in animals much oxygen con- 

 stantly enters the animal, and much car- 

 bon dioxide is given off. The breathing in 

 of oxygen is followed by oxidation of 

 food all through the animal's body. In 

 animals, cellular respiration is the term 

 applied to the movement of oxygen into 

 a cell, the oxidation of food, and the 

 passage of carbon dioxide from the cell. 

 In plants there are no breathing organs 

 and there is no active breathing, but it 

 is important to remember that there is 

 respiration and that when oxygen is not 

 already present in the plant it diffuses 

 inward, and that when the carbon diox- 

 ide is not used in makinjj su^ar, it dif- 

 fuses outward. 



Respiration and photosynthesis. The 

 exchange of gases that results from respi- 

 ration and photosynthesis must not be 

 confused with the processes themselves. 

 The two processes are altogether dif- 

 ferent. See the table on page 145. 



What part do leaves play? ^()u liavc 

 seen that leaves play a very important 



Is Made by Green Pkfits unit hi 



part in the life of the plant. The cells 

 in the inside of the blade of the leaf con- 

 tain chloroplasts. Chloroplasts are tiny 

 bodies of living matter containing chloro- 

 phyll. Through the agency of the chlo- 

 rophyll, sugar is made. The sugar can 

 change to starch. It can also combine 

 with mineral compounds, making pro- 

 teins. Sugar is also converted into fats. 

 These later changes can take place in 

 other parts of the plant as well as in the 

 leaves. But photosynthesis takes place 

 only in the green parts of the plant; this 

 means that in most plants the manu- 

 facture of most of the carbohydrates oc- 

 curs in the leaves. Photosynthesis takes 

 place only in the stems of such plants as 

 the barrel cactus (Fig. 186, p. 160). Its 

 tiny leaves soon drop off. 



The manufactured protein is combined 

 with other substances and changed into 

 living protoplasm by assimilation. The 

 carbohydrates, and to some extent the 

 fats and proteins, are oxidized in respi- 

 ration. It is by this process that the plant 

 gets the energy needed for its activities. 



How green plants are of importance in 

 the world. You have seen that green plants 

 make their own food. But they do far 

 more than this. They make the food used 

 by the whole world of animals and non- 

 green plants. Carbohydrates are made 

 only by plants containing chlorophyll; 

 proteins are made only by plants. Ani- 

 mals and nongreen plants are completely 

 dependent on green plants for their food, 

 and that means for their energy supply, 

 too. Green plants transform the radiant 

 energy of sunlighr into the chemical 

 energy in food. Without chlorophyll this 

 energy transformation could not have 

 taken place. Green plants keep storing 



