42 PHYSIOLOGY OF NUTRITION 



chemistry of this photosynthetic process is still to be worked out. — More than 90 per 

 cent, of the dry weight of the plant is derived from the carbon dioxide and water 

 used in the process here considered; the rest is derived from mineral salts absorbed from 

 the soil solution. 



7. Assimilation of Solar Radiant Energy by Green Plants. — The formation of 

 carbohydrates in green plants necessarily results in the storage of potential energy, in 

 an amount equivalent to the energy that would be freed by the complete oxidation 

 or burning of the carbohydrates formed. The fuel values of wood and coal are pro- 

 portional to the potential energy stored in these substances and set free when they are 

 burned. This energy is a part of that which was absorbed from sunlight when the 

 plants from which these fuels have been derived were growing. The stored solar 

 energy of coal has lain dormant for ages, that of wood generally for years. Cal- 

 culations indicate that 2.2-3.6 gram-calories of energy is stored for each gram of 

 carbon dioxide decomposed in photosynthesis. Experiments have shown, however, 

 that plants accumulate, as potential energy in their carbon compounds, less than 0.5 

 per cent, of the radiant energy that reaches them as sunlight. 



8. Influence of Conditions on Carbohydrate Photosynthesis. — Internal conditions 

 influencing the rate of carbohydrate photosynthesis are: (a) the amount of chlorophyll 

 present; (b) anatomical and histological structure, especially arrangement and size 

 of stomata; (c) condition of stomata — whether open, closed, partly closed, etc.; (d) 

 turgor condition— whether the leaf is wilted, etc. (this is perhaps covered by c) ; (e) 

 the rate at which products of the process leave the leaf; (/) the ability of the leaf to 

 absorb light (may be included under b) ; (g) leaf temperature. 



External conditions influencing the rate of this process are: (a) the rate of supply of 

 carbon dioxide; (b) the quality — wave-lengths — of the light received; (c) the rate of 

 light-energy absorption — intensity of each group of wave-lengths and time during 

 which the leaf is exposed to them; (d) the temperature of the surroundings — which 

 mainly controls leaf temperature; (e) other external conditions whose influence is not 

 yet so well understood. 



9. Nutrition of Green Plants by Organic Compounds. — Some green plants (as, for 

 example, the insectivorous forms, Drosera, Nepenthes, Dionaea, etc.) are able to 

 absorb considerable amounts of ready-made organic compounds, etc., from the sur- 

 roundings. Many other green plants have this ability to some degree, especially 

 parasites and saprophytes. Of course the non-chlorophyll-bearing parts of green 

 plants regularly derive their carbohydrates from the tissues that bear chlorophyll. 



