NATURE OF PLANTS 23 



periods without absorbing oxygen from the air and on the other 

 hand in some plants CO2 is not always given off because the 

 decomposition is not carried so far as to result in the formation 

 of so simple a product. We eat in order to gain possession of 

 the energy locked up in the foods. We breathe in order that the 

 oxygen may assist in the decomposition of these compounds and 

 set free this energy which gives us power to work and move 

 and keep our bodies warm. The plant lives in the same way. 

 It only differs from the animal in that it has the added power to 

 build up the complex food compounds from crude material. 

 By the decomposition of the products formed from these foods it 

 gains energy to grow and carry on its other vital functions. 

 The work of respiration is carried on more economically by green 

 plants than by animals since in the animal the CO2 escapes in the 

 breath as a waste product, while the plant uses this CO2 during 

 the day time for the construction of foods. Consequently the 

 escape of CO2 can only be observed during the night and cannot be 

 detected in the light unless an examination of very rapidly grow- 

 ing organs be made. If a jar be nearly filled with opening buds 

 of dandelions or rapidly growing shoots and then closed air tight, 

 sufficient CO2 will be respired in a few hours to extinguish a light 

 that is lowered into the jar. In such instances as these very 

 rapid respiration is necessary to furnish the required energy for 

 growth and the volumes of CO2 expired exceed many times the 

 volume of CO2 utilized in photosynthesis. A handful of ger- 

 minating peas or beans placed in a closed jar for a few hours better 

 illustrates the giving off of CO2 because here there is no green 

 tissue to absorb any of the CO2. Plants are often considered 

 unhealthf ul in sleeping rooms at night because of their exhalation 

 of CO2. It is well to remember that the amount of CO2 expired 

 by a plant is small and that a gas jet would furnish more CO2 to 

 the air than a window full of plants. A square meter of leaf 

 surface gives off about .12 gm. (60 c.c.) of carbon dioxide per 

 hour at 20° C. 



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 



