82 PLANT GROWTH 



by the large yields of crop plants. Occasionally some prun- 

 ing may be necessary to allow adequate light to reach all the 

 leaves. A number of good soil practices will favor root 

 growth and increase the photosynthetic rate. 



Shade-loving plants differ from sun plants in their struc- 

 ture and in their ability to use sunshine. They usually have 

 thinner leaves, fewer chloroplasts, but larger leaves. The 

 chlorophyll appears to be more economical in its use of sun- 

 shine or diffused light when the intensity is low. Many of 

 the ferns and other shade-loving plants may reach their 

 maximum rate of photosynthesis with a light intensity of less 

 than 40 per cent of full sunshine, but in a stronger light their 

 transpiration becomes excessive. Such plants are able to 

 store food and make normal growth in the shade. If a sun- 

 loving plant is grown in the shade its leaves are larger and 

 thinner than normal, in fact it develops the structure charac- 

 teristic of shade leaves, but its internodes are longer and its 

 rate of photosynthesis is too low to produce food for ade- 

 quate storage. This difference in storage material of sun 

 plants and shade plants grown in the shade enables the 

 shade-loving plants to crowd out the sun plants. 



The amount of carbon dioxide in the air is very small in 

 comparison with the other components. 



Average percental composition of air: 



Nitrogen 78.0 



Oxygen 21.0 



Carbon dioxide 0.03 



Other gases 0°' 



It will be seen that the plant must get its carbon dioxide 

 from air in which there are only three parts in 10,000. It has 

 been shown that by increasing the concentration of carbon 

 dioxide the rate of photosynthesis under most conditions will 

 increase, as the concentration of carbon dioxide is increased 

 to at least ten times the atmospheric content. For this 

 reason it has been felt that the total world rate of photo- 



