28 



PHYSIOLOGY OF NUTRITION 



upward, each form a plate of cells lying at right angles to the direction of the 

 impinging light. Each cell of this plate has the form of a lens and the chloro- 

 plasts lie in the prolonged basal region. Acting like biconvex lenses, these 

 cells concentrate the light of the half-dark cave sufficiently to allow carbon 

 dioxide decomposition by the chloroplasts. A part of the light is reflected, 

 thus rendering the protonema luminous. 



In general, plants are adapted to the minimum of available light (Wiesner, 

 Liubimenko). In heliophilous plants (which thrive best in bright sunshine) 

 the rate of carbon dioxide decomposition increases continuously with increase 

 in light intensity; 3 on the other hand, for heliophobous plants (which thrive 

 in shade or in regions of low light intensity) there exists an optimum light 

 intensity, and any increase beyond this optimum results in a decrease in the 

 amount of carbon dioxide decomposed. This difference is related to the 

 different amounts of chlorophyll contained in the two kinds of plants. Liubi- 

 menko was able to show that heliophobous plants are richer in chlorophyll 

 than are heliophilous ones. Within limits, the greater the amount of light and 



the higher the temperature, the smaller is the amount of 

 chlorophyll formed by the plant. 



§6. Products of Photosynthesis. 1 — The simplest equa- 

 tion that may represent the exchange of gases in photo- 

 synthesis is CO2 = C + 2 . The carbon is retained by the 

 plant, combined with other elements in the form of organic 

 substances. The question now arises as to what are to be 

 considered as the first products of photosynthesis. The 

 investigations of Sachs 2 showed that the first visible product 

 is starch. If leaves are kept for several days in darkness 

 the starch completely disappears from the chlorophyll 

 bodies, and if the leaves are then returned to light starch 

 soon appears again. Small traces of starch may be recog- 

 nized by the method of Bohm, whereby leaves are first 

 decolorized by alcohol and then treated with caustic 

 potash and iodine solution; the starch grains, greatly swollen 

 by potassium hydroxide, are stained by iodine and thus 

 become visible. If a part of the leaf is covered with tinfoil 

 before it is exposed to light, and if, after the exposure, the 

 leaf is decolorized with alcohol and then treated with iodine, the portion that 

 was shaded becomes yellowish brown, while the rest of the leaf is blue or 

 black, according to the amount of starch present (Fig. 17). The experiment 



Fig. 17. — Accu- 

 mulation of starch 

 in the illuminated 

 portion of a leaf. 

 The light-colored 

 portion was shaded 

 by tinfoil and the 

 starch has been 

 stained by iodine. 



1 Brown, H. T., and Morris, G. H., A contribution to the chemistry and physiology of foliage leaves. 

 Jour. Chem. Soc. London 63 : 604-677. 1893. 



2 Sachs, J., Ueber den Einfluss des Lichtes auf die Bildung des Amylums in den Chlorophyllkornern. 

 Bot. Zeitg- 20: 365-373. 1862. Idem, Ueber die Auflosung und Wiederbildung des Amylums in den 

 Chlorophyllkornern bei wechselnder Beleuchtung. Ibid. 22 : 289-294. 1864. 



« It is not to be understood that there are no optimum light intensities for carbon-dioxide 

 decomposition in plants that grow best in bright sunshine, only that such optima are markedly 

 higher than those for plants that grow best in shade. — Ed. 



