ASPECTS OF SALT ABSORPTION IN CELLS 103 



intra-cytoplasmic component. The bulk of the sak in the algal cell 

 occurs in the third fraction which exchanges very slowly (apparent 

 half time = 40 days) and is probably contained in the central vacuole. 

 Both the outer and inner protoplasmic membranes seem to be 

 rather impermeable in this organism (see Fig. 35a). 



Briggs and Pitman (1959) reported that the non-free space in 

 red beet cells can similarly be separated into two components, a 

 smaller fraction in which 50 per cent, of the ions exchange in 1 hr 

 and a larger fraction with an apparent half time of exchange of 

 about 1000 hr. 



A somewhat puzzling observation, in view of the results just 

 described, is the ready exchangeability of the bulk of the salt, in 

 fronds of some sea- weeds. Scott et al. (1957), for example, observed 

 that nearly 90 per cent of the sodium in Ulva lactuca fronds exchanges 

 with sodium ions from the medium within 5 sec. The speed of this 

 exchange is difficult to reconcile with accumulation of ions in 

 vacuoles assuming that the cytoplasm has a similar permeability in 

 all plant cells, and it seems likely that most of the sodium in this 

 tissue is associated with extracellular mucilage and cell walls. 

 Potassium ions exchange less rapidly than sodium ions in Ulva 

 lactuca, but more readily than the bulk of potassium in higher plant 

 cells and in coenocytes. They are lost quickly from Ulva fronds in 

 darkness and reabsorbed upon illumination (Fig. 15b, p. 49). In 

 contrast, salts accumulated in the vacuoles of higher plant cells are 

 retained by the cytoplasm, even under conditions of reduced 

 metabolism. It is possible that a large fraction of the potassium 

 content of Ulva exists in the mucilaginous cell wall and in meta- 

 bolically-bound forms in the cytoplasm, while relatively little is 

 accumulated in vacuoles, which in any case are inextensive in this 

 plant. The situation may thus be comparable to that found in 

 bacteria and other micro-organisms (see pp. 97-8). MacRobbie and 

 Dainty (1958b) distinguished rapidly and more slowly exchanging 

 components in the salt content of the red alga, Rhodymenia palmata. 

 Interpretation of their results in terms of the location of each 

 component is difficult because of the structural complexity of this 

 plant. 



More investigations of the salt relations of relatively non- 

 vacuolated cells might do much to clarify the mechanisms of ion 

 absorption and retention in cytoplasm. Apart from micro-organisms, 



