154 



MINERAL SALTS ABSORPTION IN PLANTS 



normally grows more deeply submerged, and hence at lower light 

 intensities than V. macrophysa but plants growing in smoother 

 water tend to contain more potassium relative to sodium than those 

 exposed to rough seas. There is tendency in both species for the 

 potassium to sodium ratio to diminish with age. 



The older physiologists, including Osterhout (see p. 76), believed 

 that the low concentration of sodium relative to potassium in 

 Valonia coenocytes is due to the slow rates at which sodium is 



o 



D 



O 



O 

 (J 



0-6 

 0-5 



0-4 

 0-3 

 0-2 

 0-1 

 



No* 



Mg** Ca"^ CL- SOJ 



Fig. 47. The approximate concentrations of the most abundant ions in 



sea-water. 



taken up. Another possibility is that the low internal sodium 

 concentration is maintained at least partly by active extrusion such as 

 has been demonstrated in Ulva lactuca. Scott and Hay ward (1953, 

 1954) found that Ulva cells maintain a high potassium to sodium 

 ratio only in the light, and when the thallus is transferred to darkness, 

 potassium is gradually released and sodium taken up as respiratory 

 substrates become depleted. Upon subsequent illumination, sodium 

 is excreted and potassium accumulated until the normal cation 

 balance is attained (Fig. 15b, p. 49). When a tissue has been leached 

 of salt by prolonged washing in sucrose, isotonic with sea-water, 

 and is then transferred to sea-water, the cells rapidly return to 

 equilibrium by uptake of both potassium and sodium. The 

 mechanisms of absorption of the two ions seem to be independent, 

 because uptake of potassium is unaflFected by the concentration of 

 sodium in the medium. Differences in the sensitivity of the two 

 mechanisms to metabolic inhibitors have also been noted. Sodium 



