20 MINERAL SALTS ABSORPTION IN PLANTS 



algae, particularly Valonia macrophysa, Valonia ventricosa and 

 Halicystis spp. which grow in warm seas, have attracted the 

 attention of plant physiologists interested in salt accumulation. One 

 reason for this is the relatively large size of coenocytes (a single 

 coenocyte in Valonia, for example, may have a volume of several 

 cm^) which makes it possible for uncontaminated vacuolar sap to 

 be extracted for detailed chemical analysis (Fig. 2, p. 11). Other 

 manipulations are also feasible, which are impossible or difficult 

 to accomphsh with vacuolated cells; for example, separation of the 

 cellulose wall from the protoplast is possible, and measurements of 

 electrical potential differences and resistances between the medium 

 and vacuolar sap can be made. 



There is an unfortunate tendency among plant physiologists to 

 homologize the structure of a coenocytic alga with that of a single 

 vacuolated cell of higher plants and even to refer to a coenocyte as 

 a "cell". In fact, coenocytes have probably arisen from multi- 

 cellular tissues by disappearance of intervening walls, and are 

 analogous rather than homologous with cells. In some genera of 

 coenocytic algae, e.g. Dictyosphaeria, some of the cross walls are 

 retained, dividing the protoplasm into multinucleate compartments 

 across which salts are transferred in a manner reminiscent of 

 secretion through multicellular membranes in animals. These 

 remarks are not meant to imply that the mechanism of salt accum- 

 ulation in the sap cavity of a coenocyte is necessarily different from 

 that involved in parenchyma cells, but care should be taken when 

 comparing observations made with the two kinds of material, and 

 to avoid confusion, the correct nomenclature should be used. 



8. Mycelial Fungi 



The mechanism of salt uptake by fungal hyphae has hardly been 

 studied, and an extensive investigation might profitably be under- 

 taken with this material. The fact that the guttation fluid from 

 Pilobolus sporangiophores contains salts at a high concentration 

 suggests that fungi probably accumulate ions in the central cavity 

 of the hyphae, where they assist in the maintenance of turgidity. 

 It would be interesting to examine the extent to which the mechanism 

 involved resembles that of accumulation in the vacuoles of 

 parenchyma cells. Elucidation of the salt relations of fungi may 

 improve our understanding of the manner in which the fungal 



