718 PLANT GROWTH AND PLANT COMMUNITIES 



may sometimes be the limiting factor in photosynthesis. The corn 

 leaves considered in this paper, losing 0.01 gm. of water per sq. cm. 

 per hour in an external relative humidity of 50 per cent might be ex- 

 pected to allow the passage of about 1.3 X lO"'* gm. of carbon dioxide 

 from an external CO2 concentration of 0.03 per cent and an internal 

 CO2 concentration of zero. This would be about the same as the rate of 

 CO2 uptake by corn observed by Moss, Musgrave, and Lemon (1960) 

 at a light-intensity of 6,000 foot-candles. Moss and Musgrave found 

 that under these conditions increases in the amount of CO2 in the air 

 caused an increase in the photosynthetic rate, so it appears that the 

 transfer of COo was limiting. Unfortunately, the plant has no mecha- 

 nism by which it can decrease the diflPusion-path resistance to carbon 

 dioxide without reducing the resistance of the same path to water va- 

 por, although any temperature diflference between the leaf and the air 

 could be expected to work in opposite directions on the two processes. 



Flow of ions 



In spite of the difficulties in measuring and interpreting salt move- 

 ment in plants, all such movement must conform to the General Trans- 

 port Law. Because of the obscurity of the mechanisms involved, appli- 

 cation of this law to such movement has not been as useful as the appli- 

 cation to water movement. Adsorption of ions to plant materials and 

 movements by protoplasmic streaming further complicate the picture. 

 Ions have the same three root paths available to them as were discussed 

 for water, but the transport characteristics of the paths may be much 

 different for ions than for water, and the relative importance of the 

 three paths is currently the subject of active investigation and debate. 



Path A, directly through the cytoplasm and vacuoles of the cells, 

 is undoubtedly the path taken by some of the ions. It is known that the 

 cells can store salts in solution in their vacuoles in concentrations far 

 greater than the concentration in the external medium. In terms of the 

 General Transport Law, this means that the potential barrier at the 

 vacuolar membrane (or somewhere in the cytoplasm) must be high 

 enough to prevent the outward flow of these ions. It is also known that 

 the plant can drastically deplete the nutrient solution of its salts, so 

 some region in the cytoplasm or vacuolar membrane must act as a sink, 

 or place at which the potential of the ions is lowered. Thus there must 

 exist a polarity within the cell; further, there must be a source of energy 

 to establish such a concentration gradient. Both of these conditions can 

 be fulfilled by some sort of ion-carrier system with appropriate concen- 

 tration gradients in combined and uncombined carriers maintained by 

 metabolic energy. The amount of energy required per ion depends 



