ASSIMILATION OF CARBON 41 



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

 C40H56O2. 



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. A 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 

 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 (CH 2 0), the latter being polymerized, with the formation of dextrose (C6H 12 6 ). 

 The hypothesis is represented by the equations: (1) C0 2 + H 2 = CH 2 + O2 

 and (2) 6CH 2 = CeH 12 06. Traces of formaldehyde 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 antecendent decomposition 

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



