10 MINERAL SALTS ABSORPTION IN PLANTS 



An early idea was that salts were absorbed passively with the water 

 that enters the plant. Such a view was expressed in Strasburger's 

 well-known textbook and still finds some support today (see 

 Chapter 7, pp. 116-7. A first indication of the inadequacy of this 

 hypothesis came from observations of De Saussure that the salt 

 composition of plants differs from that of the environment, and it 

 became necessary to search for an alternative mechanism. The 

 rapid increase in knowledge of physical chemistry during the 

 nineteenth century provided a stimulus for most of the suggestions 

 that were made. Mulder {Physiologie Chemie, 1851) argued for the 

 importance of osmosis, that is, diff'usion of substances across 

 membranes in response to concentration gradients, pointing out 

 that chemical transformation of particular molecules inside the 

 protoplasm could result in continuous absorption, and also account 

 for selectivity. This idea was supported by Sachs (1875) and Pfeffer 

 (1900) who recognized, however, the inadequacy of diffusion and 

 osmosis for transport of substances within plants at the rates 

 observed. They emphasized the role of the transpiration stream in 

 causing the rapid longitudinal movement of salts from roots to shoots. 



The problem of the structure and properties of membranes, both 

 living and non-living, began to interest physical chemists during the 

 latter part of the nineteenth century. Traube (1867) suggested that 

 membranes contain pores of fixed sizes through which molecules 

 may pass if their dimensions are not too great (molecular sieve 

 hypothesis). Overton (1895) on the other hand, demonstrated the 

 importance of lipid solubility in determining the rate at which 

 substances are transferred across membranes (lipid hypothesis). At 

 first, the permeability of cytoplasm was looked upon as a passive 

 property, comparable to that of non-living membranes, but Overton 

 realised that the metabolism of the cell might play some part 

 ("adenoid activity"). With remarkable insight, Pfeffer (1900) 

 asserted that living organisms may possess the ability to transport 

 substances across membranes in a particular direction, and to 

 induce movement from cell to cell, in the absence of concentration 

 gradients. He suggested that chemical combination with cell 

 constituents may be involved in these processes, and thus conceived 

 the idea of "carrier molecules" which is now in vogue. 



Pfeffer's suggestion was received without enthusiasm, and for 

 many years afterwards physical mechanisms were continually 



