CHAPTER 2 



PLANT STRUCTURE IN RELATION TO 

 PHOTOSYNTHESIS 



The structure of the plant will determine the accessibility 

 of the reactant carbon dioxide to the assimilatory cells. For 

 synthesis of one gram in the form of hexose the land plant 

 consumes carbon dioxide equal to that in 2,500 litres of air; 

 measurements indicate that, at the best, the plant utilizes 

 only two thirds of the carbon dioxide of the air and thus the 

 actual volume of air required is greater than this. The con- 

 centration of carbon dioxide in the atmosphere is small com- 

 pared with that of oxygen (0-03% against 21-0%); yet the 

 rate of photosynthesis of a land leaf under natural conditions 

 may be some 10 times greater than the rate of respiration. 

 The rate of these processes may under any set of conditions 

 be determined by either physical or chemical factors. In this 

 chapter we shall consider only the physical factors affecting 

 the supply of reactant to the green cells, e.g. the diffusion of 

 carbon dioxide into the plant. 



The diffusion path for carbon dioxide into the cells of the 

 leaf of a higher plant is complex. Carbon dioxide diffuses 

 from the air outside the leaf up to and through the stoma, 

 across the intercellular space, and then through the wet cell 

 wall up to the centre where reaction occurs. The rate of dif- 

 fusion is proportional to the difference in concentration of 

 carbon dioxide between the two ends of the path, to the area 

 of the path, and inversely proportional to the length of path. 

 It is also directly proportional to the diffusion constant, a 

 characteristic of the substance diffusing and of the medium 

 through which diffusion occurs. 



