40 PHYSIOLOGY OF NUTRITION 



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: (i) light (within the limits of the visible spectrum and with different intensities 

 for different kinds of plants); (2) temperature (from about o°C. to about 45°C. as 

 general limits; the range is usually narrower, 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 40 H 56 . 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 

 plants. — Xanthophyll resembles carotin but contains some oxygen; it has the formula 

 4oCH 56 09. 



5. Influence of Light in Carbohydrate Photosynthesis. — Light impinging on leaves 

 is partly reflected, partly absorbed, and partly transmitted. Only that which is 

 absorbed can influence chemical processes within the leaves. The absorbed portion 

 may have various qualities (according to the proportions of the different ranges of 

 wave-length that are present) and various total intensities. The range of wave- 

 lengths approximately corresponding to our visual range of red and orange appears 

 usually to be most effective in furnishing energy for photosynthesis, but the rest of 

 the visible range of wave-lengths is not without effect. The proportional distribution 

 of total light energy among the several ranges of wave length varies greatly in nature. 

 When the relations between light quality and carbohydrate photosynthesis are to be 

 dealt with, it is necessary to consider the energy-supplying power of any wave-length 

 range of absorbed light. It has been suggested that the rate of the process may be 

 proportional to the energy value of the absorbed light, other conditions being adequate 

 and constant throughout the series of comparisons. The absorbing power of chloro- 

 phyll-bearing tissue, for the different ranges of wave-lengths, is greatly influenced by 

 the amount of chlorophyll present and by the presence of pigments other than chloro- 

 phyll — also by the cell structures of the tissues. — Considering simply the total intensity 

 of sunlight, carbohydrate photosynthesis proceeds with intensities between a minimum 

 and a maximum, with an optimum intensity somewhere within the range. Shade- 

 plants (as beech) have a low range of intensities for the process, while sun-plants (as 

 pine) have a high range. Cave mosses thrive with very weak illumination. 



6. Products of Carbohydrate Photosynthesis. — If a living green plant that forms 

 starch be kept in darkness till all starch has disappeared from the chlorophyll-bearing 

 cells, and if it be then exposed to suitable light, starch grains soon appear in the cells. 

 But starch is not the first product of the photosynthetic process, for starch is formed 

 from a water-soluble sugar (such as dextrose), not directly from carbon dioxide and 

 water. There are plants that do not form starch, and these show an increased amount 

 of sugar when they are brought into light after a prolonged period in darkness. A 

 supply of carbon dioxide is of course necessary, in the surrounding air, and the form- 

 ation of sugar or starch proceeds parallel to the absorption of carbon dioxide by the 

 plant in this kind of a test. Of course the active cells are plentifully supplied with 

 water, which is the other necessary material. Besides sugar, a prominent product of 



