ASSIMILATION OF CARBON 4 1 



this process is oxygen, most of which escapes from the green tissues into the. 

 surroundings. 



6a. Chemistry of Carbohydrate Photosynthesis. — Baeyer's hypothesis supposes 

 that carbon dioxide and water are decomposed, that some free oxygen is produced, and 

 that the remaining carbon, hydrogen, and oxygen are combined to form formalde- 

 hyde (CH2O), the latter being polymerized, with the formation of dextrose (C 6 Hi 2 6 ). 

 The hypothesis is represented by the equations: (1) CO2 + H 2 = CH 2 + 2 

 and (2) 6CH2O = C 6 Hi 2 6 . Traces of formladehyde have been found in green 

 tissues, and green plants in light have been experimentally shown to be able to increase 

 their carbohydrate content when supplied with this substance as the only source of 

 carbon. But formaldehyde is a violent poison and can never accumulate considerably 

 in living tissues. It is supposed that this substance generally polymerizes as rapidly 

 as it is formed. If the hypothesis is true, light appears necessary for the polymeriza- 

 tion of formaldehyde, as well as for its formation and for the antecedent decomposition 

 of carbon dioxide and water. Many other hypotheses have been suggested, and the 

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

 controls leaf temperature; (c) 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 carbohydrates, etc., from the surround- 

 ings. Many other green plants have this ability to a smaller degree. Of course the 

 non-chlorophyll-bearing parts of green plants, regularly, derive their carbohydrates 

 from the tissues that bear chlorophyll. 



