234 Plant Biology 



varies directly with the energy absorbed from the Ught regardless of the 

 wave length." The physicist, Langley, determined the distribution of 

 energy in the spectrum as follows: 



SPECTRUM REGION PERCENTAGE OF TOTAL ENERGY 



Infrared 62—63 

 Visible spectrum 37.0 

 Uhraviolet 0^6 



About 63 per cent of the total energy has no value in the photosyn- 

 thetic process, because none of the infrared waves are used. The rela- 

 tion between light and photosynthesis is considered in another part of 

 this chapter. 



Chlorophyll also possesses the optical property of fluorescence. In 

 reflected light it appears blood red due to the fact that part of the light 

 waves falling on it are transformed and reflected with an altered wave 

 lens:th. 



'O" 



Influential Factors in Photosynthesis. — 



1. The Carbon Dioxide Supply: The quantity of carbon dioxide in 

 the air is a very important factor in photosynthesis. The average 

 amount in the air is about three parts per 10,000 (0.03 per cent). This 

 amount is usually too small for a maximum of photosynthesis because 

 experiments show that many of the common plants could use efficiently 

 up to 1 per cent. Certain plants might even use higher concentrations- 

 Increased carbon dioxide must be accompanied by corresponding in- 

 creases in temperature and illumination if maximum use of the gas is 

 to be made. Approximately 50 per cent of the dry weight of a plant 

 body is composed of carbon which for the most part must come from 

 the air. The application of additional amounts of carbon dioxide to 

 such crops as tomatoes, potatoes, beets, and carrots increased their yield 

 from 30 to 300 per cent. A tree with a dry weight of 1,000 pounds 

 must secure 500 pounds of carbon from approximately 1,427,000 cubic 

 yards of carbon dioxide from the air. Under natural conditions, the 

 amount of carbon dioxide in the air is probably a limiting factor in the 

 rate of photosynthesis. 



2. Quantity and Quality of Light: The quantity (intensity), quality 

 (wave lengths), and the duration of light all affect the rate of photo- 

 synthesis. Certain plants apparently require small amounts of light for 

 the process. A lighted match held for one second 10 cm. away from a 

 green alga (Chorella sp.) will initiate the process with the evolution of 



