108 MINERAL SALTS ABSORPTION IN PLANTS 



Chloroplasts are surrounded by a distinct membrane, and because 

 they swell in media of low osmotic pressure, it has been inferred 

 that they behave as osmometers. The evidence that the surface 

 membrane is selectively permeable is as yet inconclusive. Diamond 

 and Solomon (1959) have demonstrated that the chloroplasts in 

 Nitella cells do not represent a separate phase in the photo- 

 plasm as far as the exchangeability of potassium ions is concerned. 

 Gross analyses of isolated chloroplasts indicate that their salt 

 content is often not strikingly different from that of the cell as a 

 whole. Alkali and alkali earth cations are somewhat less abundant in 

 chloroplasts than in mitochondria or vacuoles, and this may be 

 related to the fact that they exist mainly as free ions in aqueous 

 solution. On the other hand, ions such as phosphate, iron and zinc 

 which become readily incorporated into organic complexes tend to 

 be more abundant in chloroplasts than elsewhere. Amon (1955) 

 has shown that in addition to being capable of photolysis of water 

 and reduction of carbon dioxide, isolated chloroplasts in the light 

 incorporate inorganic phosphate into organic phosphorus com- 

 pounds ("photophosphorylation"). It would be interesting to 

 examine the possibility that photophosphorylation can promote 

 absorption of other ions than phosphate into chloroplasts and intact 

 cells. 



B. Transport from Cell to Cell 



The protoplasm in a multicellular organism is continuous from 

 one cell to another via protoplasmic connections ("plasmadesmata") 

 (Fig. 5g, p. 19), forming a "symplast", and it is apparently through 

 these that ions are transported from cell to cell. It is probable that 

 some salt is carried bound to the membranes of the endoplasmic 

 reticulum which are continuous between cells, and in constant 

 movement (cf. pp. 105-6). The large central vacuoles are not contin- 

 uous from cell to cell, and are unlikely to have a direct influence on 

 movement of salts. Small vacuoles, however, may move through the 

 protoplasmic connections and could act as vehicles in salt transport. 



Movement of ions through parenchyma has been studied in 

 leaves of a water plant, Vallisneria spiralis by Arisz, and his associates 

 in the Netherlands (Arisz, 1953; Arisz and Schreuder, 1956; Arisz 

 and Sol, 1956). It was found that when excised leaf segments are 

 placed on the surface of agar containing sodium or potassium 



