102 PHOTOSYNTHESIS 



chlorophyll-content and photosynthetic activity. Willstatter has shown 

 that the effect of the internal factor is increased by raising the temperature. 

 However, an increase in the activity of the internal factor is of influ- 

 ence on photosynthesis only in leaves with a sufficiently high chlorophyll- 

 content. The latter question is discussed in another section of this book. 



It is a matter of common observation that some plants thrive in the 

 shade, protected from the direct rays of the sun, while others do best under 

 conditions of direct insolation. Undoubtedly the causes which underlie 

 such an adaptation to environmental conditions are complicated. Yet 

 under these different conditions of illumination the respective plants are 

 capable of carrying on their photosynthetic activity with a total net 

 gain to each in dry matter. Boysen-Jensen ^^ has made a study of the 

 photosynthetic activity of some typical "shade" and "light" plants with 

 special reference to their gain in dry matter, i.e. "the amount of dry matter 

 produced in unit time, calculated in per cent of dry matter in the plant in 

 question at the beginning of the experiment." As an example of a light 

 plant Sinapis alba is taken, and as a shade plant Oxalis acetosella. In 

 Boysen-Jensen's experiments the former were grown in "full daylight" 

 and the latter in "a light intensity as small as possible." His own con- 

 clusions are as follows : "In Sinapis the intensity of COo assimilation is 

 very great, rising to at least 6 mg. CO2 per 50 cm.^ per hour at 20°. Also 

 the respiration in the leaves is great, about 0.8 mg. CO2 per 50 cm.^ per 

 hour at 20°. The point of equilibrium between CO2 assimilation and 

 respiration lies at a light intensity of 1.0 (Bunsen units X 100). The de- 

 velopment of a Sinapis plant is very quick. In four weeks the dry matter 

 content rises from 0.5 g. to 38 g. per 100 plants. In favorable conditions 

 the daily per cent production of dry matter can be estimated as about 15. 



"In Oxalis the maximal intensity of COo assimilation is very small, 

 alx)ut 0.8 mg. CO, per 50 cm.- per hour at 20°. Also the respiration 

 of the leaves is small, about 0.1-0.2 mg. COo per 50 cm.=^ per hour at 20°. 

 The point of equilibrium between COo assimilation and respiration lies at 

 a light intensity of 0.2. The daily per cent production of dry matter is 2.1." 



From these experiments it is apparent that, as nearly as two such plants 

 can be compared, the one growing in the sunlight does more photo- 

 synthetic work than the other, a conclusion which is not surprising. In 

 both cases respiration took about 13 per cent of the material synthesized. 

 Boysen-Jensen also calculated from .Combes' ''^ data the per cent produc- 

 tion of dry matter of a number of rapidly growing plants. The results 

 are given in Table 18. 



Of course, the foregoing results contribute very little to the question 

 of the efficiency of different plants in utilizing light for photosynthetic 

 work. This question is discussed in the section on energy relations. Some 



"Boysen-Jensen, Bot. Tidsskrifl, 36, 219 (1918). Johansson. Svcns K. Bot. 

 Tidsskrift, 17, 215 (1923). 



•"Combes, Ann. d. Sci. nat., IX Ser. Bot., 11, 75 (1910). See also Weis, R, 

 Compt. rend., 137, 801 (1903). Weber, Arb. Bot. Inst. Wiirzburg, 2, 346 (1879). 



