40 PHYSIOLOGY OF NUTRITION 



(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, excepting what is used in respiration. The carbon dioxide decomposed in 

 photosynthesis may come partly from the soil, dissolved in the water that is absorbed, 

 as well as from the air, and a small part of it is derived from the respiratory process 

 within the plant. 



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. 

 Thev 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 O, 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 

 680/4U. (red) is most completely absorbed. With wave-lengths shorter than about 

 475MM (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 are chemically some- 

 what related to hemoglobin (which occurs in red blood-corpuscles of animals) ; they 

 give several of the same decomposition products. 



For the formation of chlorophyll in leaves, etc., the following conditions are essen- 

 tial: (1) light (within about the limits of the visible spectrum and within a certain range 

 of intensities, this range differing considerably for different kinds of plants); (2) tem- 

 perature (from about o°C. to about 45°C. as general limits; the range is usually nar- 

 rower, differing for different kinds of plants); (3) iron (but the supply must be very 

 small or poisoning results); (4) oxygen; (5) salts derived from the soil (containing 

 K, Ca, Mg, N, P, S) ; (6) water-soluble carbohydrates. 



4. Pigments Accompanying Chlorophyll. — Several other pigments accompany 

 the chlorophylls, especially carotin and xanthophyll, which are generally present in 

 cells with the green pigments, but often occur in the absence of the latter. Carotin is 

 a hydrocarbon, with the formula C 4 oH 5 e. It forms crystals that appear blue-green by 

 reflected light and orange-red by transmitted light. It is insoluble in water, readily 

 soluble in ether, carbon bisulphide, etc., and is readily oxidized. In leaves it varies 

 in amount, according to the light intensity, temperature, etc. It occurs in all parts of 



