AISSIMILATION OF CARBON BY FLAN1\^ 



193 



flow of the produces of assimilation is establishea from the 

 parenchyma cells to the veins of the leaf and further from the 

 veins through the petiole to the stems and other organs of 

 the plant. 



45. Dependence of Photosynthesis on Light Intensity. Sun 

 and Shade Plants. Culture of Plants in Artificial Light. — Light 

 being the source of energy for photosynthesis, it is natural that 

 with increased intensity of light the process also goes on at a 

 higher rate, but this does not hold true for high light intensity. 

 If, beginning with low values, the intensity of light is augmented 

 by steps representing equal energy increases, assimilation will 



c 4 



o 



"-P 

 O 



1 ^ 



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< 2 



CM 

 O 



^ 1 



760 Vzo 



Vio 



Ve 



V4 



V3 



Ligh+ Intensity 



Fig. 56. — Dependence of assimilation on the intensity of light in a light plant 

 (Nasturtium; and a shade plant (Oxalis) {ajter Lundegardh) . 



increase by about uniform steps until a certain intensity of light 

 is reached. Further increase of light intensity then induces no 

 increase of photosjnithesis. If the dependence of carbon assimi- 

 lation on the intensity of light is represented graphically, marking 

 on one axis the quantity of carbon dioxide that is decomposed 

 during a time unit, and on the other the intensity of light in 

 fractions of direct sunlight falling on the surface of the earth 

 during the noon hours of a bright summer day, it will be seen 

 that the curve of the dependence of assimilation on the intensity 

 of light will show in general, a logarithmic character (Fig. 56). 

 Not in all plants, however, is the dependence of assimilation 

 on the amount of light expressed by a curve of this nature. In 

 those peculiar to open, sunny habitats, assimilation increases 



