22 NATURE OF RESPIRATION 



exceeds many times the volume of CO 2 utilized in photosynthesis. 

 A handful of germinating peas or beans placed in a closed jar 

 for a few hours better illustrates the giving off of CO 2 because 

 here there is no green tissue to absorb any of the CO 2 . Plants 

 are often considered unhealthful in sleeping rooms at night be- 

 cause of their exhalation of CO 2 . It is well to remember that 

 the amount of CO 2 expired by a plant is small and that a gas 

 jet would furnish more CO 2 to the air than a window full of 

 plants. 



We are now in a position to understand the importance of 

 photosynthesis and respiration. Photosynthesis keeps the air 

 pure for breathing, decomposes the simple inorganic compounds 

 and recombines them into foods which represent a certain amount 

 of stored-up energy. Respiration breaks down the foods into 

 products which form the tissues of the plant and sets free the 

 energy necessary for the accomplishment of its growth and move- 

 ments. In the animal and plant the oxygen decomposes the foods 

 rather slowly and the energy set free is principally manifested as 

 heat and as the power that enables them to perform their various 

 activities. If the decomposition is sufficiently rapid light as well 

 as heat appears. This is the nature of the reaction when plant 

 tissues, such as wood, are burned. The oxygen so rapidly de- 

 composes the products which compose the wood that heat and 

 light are produced. In our fires we are dealing with the heat and 

 sunlight that have been slowly stored up by the plant. 



13. The Third Function of the Leaf, Transpiration. This 

 function refers to the giving off of water by the plant. While 

 other parts of the plant assist in transpiration, the leaf is the 

 principal organ upon which this very considerable work devolves. 

 Water is given off from the plant as a vapor and for this reason 

 transpiration is a more familiar phenomenon than photosynthesis 

 and respiration, where we are dealing with an interchange of 

 invisible gases. We see the vapor from plants growing in a 

 window precipitated on the cool window panes in the form of 

 drops. On hot summer days the leaves of plants droop. This 

 is because they have transpired so much water that their cells 

 are no longer distended by the water, consequently the cells shrink 



