86 HOW GREEN PLANTS MAKE FOOD 



Respiration by Leaves. — All living things require oxygen. It 

 is by means of the oxidation of food materials within the plant's 

 body that the energy used in growth and movement is released. 

 A plant takes in air with its oxygen largely through the stomata 

 of the leaves, to a less extent through the lenticels in the stem, and 

 through the roots. Thus rapidly growing tissues receive the 

 oxygen necessary for them to perform their work. One of the prod- 

 ucts of oxidation in the form of carbon dioxide is also passed 

 off through these same organs. It can be shown by experiment 

 that a plant uses up oxygen in the darkness ; in the light the amount 

 of oxygen given off as a by-product in the process of starch making 

 is much greater than the amount used by the plant. 



Summary. — From the above paragraphs it is seen that a leaf 

 performs the following functions : (1) respiration, or the taking in 

 of oxygen and passing ofT of carbon dioxide ; (2) photosynthesis, or 

 starch making, with the incidental passing out of oxygen ; (3) for- 

 mation of proteins, with their digestion and assimilation to form 

 new tissues ; and (4) the transpiration of water. 



Problem Questions 



1. Why is it necessary for fluids to pass up a stem into the leaves? 



2. Of what use to man is the evaporation of water from leaves? 



3. Why does the amount of transpiration vary? 



4. Explain the process of photosynthesis. 



5. In what respects is this process of value to man? 



6. Should green plants be kept in a sick room at night? In the daytime? 

 Explain. 



7. Do plants breathe ? How? 



Problem and Project References 



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



Calkins, Biology. Henry Holt and Company. 



Coulter, Barnes, and Cowles, A Textbook of Botany, Part II and Vol. II. 



American Book Company. 

 Densmore, General Botany. Ginn and Company. 

 Duggar, Plant Physiology. The Macmillan Company. 

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

 Goodale, Physiological Botany, pages 337-353 and 409-424. American Book 



Company. 

 Transeau, General Botany. World Book Company. 



