ASSIMILATION OF CARBON 39 



Experiments in which green plants were supplied with organic nitrogenous 

 compounds, in a chamber free from carbon dioxide, gave negative results. 1 



Summary 



1. Importance of Carbon Assimilation by Green Plants. — Green plants form 

 organic compounds from inorganic ones. Non-green plants and animals are unable to 

 do this and are therefore all ultimately dependent on green plants for organic sub- 

 stances. The study of plant physiology may begin by inquiring about photosynthesis 

 of carbohydrates by the green parts of plants. These organic compounds are formed 

 from carbon dioxide and water, by means of solar energy that is absorbed and trans- 

 formed in the green tissues. Carbon dioxide is of course a carbon compound, but it is 

 not combustible and is usually classed as inorganic. Combustible carbon compounds 

 derived from organisms are capable of being burned in air because they are incom- 

 pletely oxidized; when completing their oxidation these compounds absorb oxygen and 

 produce carbon dioxide and water, and this process of combustion liberates energy 

 (heat or light or both). A certain amount of sunlight energy is absorbed, and a 

 corresponding amount of oxygen is eliminated, when carbon dioxide and water are 

 combined by green plants, with the formation of carbon compounds. 



2. Exchange of Gases. — Photosynthesis is accompanied by taking in of carbon 

 dioxide and giving out of oxygen, as well as by absorption of solar energy, and the ratio 

 of the amount of absorbed carbon dioxide to the amount of oxygen eliminated in the 

 same period has been found to have a value somewhat less than unity. The process 

 results in decomposition of carbon dioxide and water, and in the union of the carbon, 

 the hydrogen, and some of the oxygen, to form carbohydrates; the rest of the oxygen is 

 given off. 



3. Chlorophyll. — The two green pigments that make it possible for carbohydrate 

 photosynthesis to occur in green plant tissues when light is properly supplied are 

 called chlorophyll, or, more correctly, the chlorophylls. Photosynthesis of carbo- 

 hydrates from carbon dioxide and water does not occur in tissues that do not contain 

 these pigments. The green pigments are named chlorophyll a and chlorophyll b. 

 They occur in green leaves in about the proportions 72 to 28, by weight. Dissolved in 

 ethyl alcohol, the first appears blue-green, the second yellow-green, by transmitted 

 light. Both are fluorescent, the first appearing blood-red, the second brown-red, by 

 reflected light. The two are alike in that each molecule contains 55 atoms of C, 4 

 atoms of N, and a single atom of Mg. The molecule of chlorophyll a contains 72 

 atoms of H and 5 atoms of 0, while that of chlorophyll b contains 70 atoms of H and 

 6 atoms of O. Iron is necessary for the formation of the chlorophylls in plants, but 

 does not occur in the pigments themselves. 



The chlorophylls absorb light more or less completely according to the wave- 

 lengths of the light that is supplied. Light of wave-lengths from about 640 to about 

 680 /л (red) is most completely absorbed. With wave-lengths shorter than about 

 475 /j. (blue to ultra-violet) absorption is almost as complete. The spectrum of 

 chlorophyll solution shows, between these two, several other ranges of wave-lengths, 

 with less complete absorption, and very strong solutions show complete absorption 

 throughout the entire range of visible light. — The chlorophylls arc chemically some- 



1 Gräfe, Victor, Untersuchungen über die Aufnahme von Stickstoffhaltigen organischen Substanzen 

 durch die Wurzel von Phanerogamen bei Ausschluss der Kohlensäure. Sitzungsber. (math.-naturw. Kl.) 

 K. Akad. Wiss. Wien n8': ii35-n.i3- 1909. 



