PLANT STRUCTURE 7 



where K is the coefficient of diffusion, A is the area of path 

 ^C is the difference in concentration in a distance dx 

 dt is a small time interval during which ds is the net 

 quantity of substance transferred. 



VARIATION IN THE DIFFUSION PATH FOR DIFFERENT PLANT 



STRUCTURES 



Organisms such as the unicellular algae and bacteria pos- 

 sess the simplest diffusion path, carbon dioxide entering 

 through the whole surface of the plant. In the case of water 

 plants the medium is buffered due to the presence of bicar- 

 bonate ions, but the concentration of free carbon dioxide 

 will be less than that in air and the volume necessary for the 

 supply of I gm. of assimilate will be slightly greater but of a 

 similar order to that for air. For an alga of small dimensions, 

 e.g. Chlorella, the diffusion path is short and, assuming the 

 diffusion constant of carbon dioxide through the cell wall to 

 be of the same order of magnitude as for diffusion in aqueous 

 solution, the concentration of carbon dioxide available to 

 the photosynthetic system approaches very near to that in 

 the environment. This remains true even for the maximum 

 observed rates of photosynthesis and in spite of the rela- 

 tively slow rate of diffusion of carbon dioxide in water, pro- 

 vided the cells do not exceed a few /n in diameter. With 

 larger algae and the higher water plants, where the whole ex- 

 ternal surface again represents the area available for entry, 

 the equivalent diffusion path is longer so^that even under 

 conditions of moderate rates of photosynthesis there will 

 be a considerable gradient of carbon dioxide between the 

 environment and the reaction centres for photosynthesis. In 

 contrast with land plants thin leaf laminae are common and 

 chloroplasts occur in the epidermal cells with fewer chloro- 

 plasts towards the centre of the tissue. The leaf of the land 

 plant shows the development of a different type of diffusion 

 path. The external surface is generally covered by a cuticu- 

 lar layer (which when present in submerged leaves is very 

 thin) and this offers considerable resistance to gaseous dif- 

 fusion. This layer is, however, broken at intervals by sto- 

 mata (Gk. stoma, a mouth) which give access to a series of 



